[dg] Update python environment config (#29290)

## Summary & Motivation

This PR makes several changes to `dg`'s venv management:

- Change the configuration schema for setting a project python
environment

Previous:

```[tool.dg.project]
python_environment = "active"
OR
python_environment = "persistent_uv"
```

Now:

```[tool.dg.project.python_environment]
active = true
OR
uv_managed = true
```

- Change the default python environment for scaffolded projects from
`uv_managed` to `active`. This means that scaffolding a new project no
longer creates a venv/uv.lock by default.
- Remove the `--skip-venv` option to project scaffolding commands. This
no longer makes sense. We never create a venv when using `active`, and
we always create a venv when using `uv_managed`.

A consequence of the switch from scaffolding uv_managed to active python
environment is that I needed to update all the guide snapshot tests to
use `--python-environment uv_managed`. In a followup I will switch back
the ones that should be using "active" mode, but adjusting the tutorials
to use "active" should go in its own PR.

Note a third `tool.dg.project.python_environment` option, `path`, will
be added in a followup:

```
[tool.dg.project.python_environment]
path = "/path/to/venv"
```

## How I Tested These Changes

Updated existing tests.

## Changelog

- Changed configuration of project Python environments. The
`tool.dg.project.python_environment` previously accepted a string,
`"active"` or `"persistent_uv"`. Now it accepts a table with one of
three keys:
    - `{active = true}`: equivalent of previous `"active"`
    - `{uv_managed = true}`: equivalent of previous `"persistent_uv"`
- Changed the default python environment for newly scaffolded projects
to `tool.dg.project.python_environment` to `{active = true}`. This means
by default, no virtual environment or `uv.lock` will be created when
scaffolding a new project (via `dg init` or `dg scaffold project`). You
can pass `--python-environment uv_managed` for the old behavior.
- Removed the `--skip-venv` flag on `dg scaffold project` and `dg init`.

Author: smackesey

examples/docs_snippets/docs_snippets/guides/components/index/2-scaffold.txt

@@ -1,4 +1,4 @@
-dg scaffold project jaffle-platform
+dg scaffold project jaffle-platform --python-environment uv_managed
 
 Creating a Dagster project at /.../jaffle-platform.
 Scaffolded files for Dagster project at /.../jaffle-platform.

examples/docs_snippets/docs_snippets/guides/components/index/7-dg-list-plugins.txt

@@ -1,5 +1,6 @@
 dg list plugins
 
+Using /.../jaffle-platform/.venv/bin/dagster-components
 ┏━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┓
 ┃ Plugin  ┃ Objects                                                                                                    ┃
 ┡━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┩

examples/docs_snippets/docs_snippets/guides/components/shell-script-component/1-dg-scaffold-shell-command.txt

@@ -1,4 +1,5 @@
 dg scaffold component-type ShellCommand
 
+Using /.../my-component-library/.venv/bin/dagster-components
 Creating a Dagster component type at /.../my-component-library/src/my_component_library/lib/shell_command.py.
 Scaffolded files for Dagster component type at /.../my-component-library/src/my_component_library/lib/shell_command.py.

examples/docs_snippets/docs_snippets/guides/dg/configuring-dg/project-config.toml

@@ -30,17 +30,20 @@ code_location_name = "my-project"
 # (string, optional) Specifies the python environment to use when spawning subprocesses
 # for this project. Subprocesses are spawned whenever `dg` needs to list
 # available component types, check definitions, scaffold new components, etc.
-# The value must be one of: 
-#
-# - "active" (default): use the currently active Python environment, i.e. the
-#   Python environment corresponding to `which python`. This may be an
-#   activated virtual environment.
-# - "persistent_uv": use a dedicated project-scoped virtual environment that is
-#   located at <project_root>/`.venv` and is managed by `uv`. Subprocesses are
-#   launched with `uv run`.
+# If this table is defined, it must have only a single key-value pair. See below for options.
 #
 # Defaults to "active".
-python_environment = "active"
+[tool.dg.project.python_environment]
+
+# (default): use the currently active Python environment, i.e. the
+# Python environment corresponding to `which python`. This may be an
+# activated virtual environment.
+active = true
+
+# - "uv_managed": use a dedicated project-scoped virtual environment that is
+#   located at <project_root>/`.venv` and is managed by `uv`. Subprocesses are
+#   launched with `uv run`. The active virtual environment is ignored
+# uv_managed = true
 
 [tool.dg.cli]
 

examples/docs_snippets/docs_snippets/guides/dg/scaffolding-project/1-scaffolding-project.txt

@@ -2,4 +2,4 @@ dg init --project-name my-project
 
 Creating a Dagster project at /.../my-project.
 Scaffolded files for Dagster project at /.../my-project.
-...
+You can create additional projects later by running 'dg scaffold project'.

examples/docs_snippets/docs_snippets/guides/dg/scaffolding-project/2-tree.txt

@@ -11,8 +11,7 @@ tree
     │       │   └── __init__.py
     │       └── lib
     │           └── __init__.py
-    ├── tests
-    │   └── __init__.py
-    └── uv.lock
+    └── tests
+        └── __init__.py
 
-7 directories, 7 files
+7 directories, 6 files

examples/docs_snippets/docs_snippets/guides/dg/workspace/5-scaffold-project.txt

@@ -1,4 +1,4 @@
-dg scaffold project projects/project-2
+dg scaffold project projects/project-2 --python-environment uv_managed
 
 Creating a Dagster project at /.../dagster-workspace/projects/project-2.
 Scaffolded files for Dagster project at /.../dagster-workspace/projects/project-2.

examples/docs_snippets/docs_snippets_tests/snippet_checks/guides/components/test_component_docs_adding_attributes_to_assets.py

@@ -35,7 +35,7 @@ def test_components_docs_adding_attributes_to_assets(
     with isolated_snippet_generation_environment() as get_next_snip_number:
         # Scaffold code location, add some assets
         _run_command(
-            cmd="dg scaffold project my-project --use-editable-dagster && cd my-project/src",
+            cmd="dg scaffold project my-project --python-environment uv_managed --use-editable-dagster && cd my-project/src",
         )
         _run_command(
             cmd="dg scaffold dagster.asset team_a/subproject/a.py",

examples/docs_snippets/docs_snippets_tests/snippet_checks/guides/components/test_components_docs.py

@@ -50,8 +50,9 @@ def next_snip_no():
         )
 
         # Scaffold project
+        # TODO: Make this use "active" python environment in docs followup
         run_command_and_snippet_output(
-            cmd="dg scaffold project jaffle-platform --use-editable-dagster",
+            cmd="dg scaffold project jaffle-platform --python-environment uv_managed --use-editable-dagster",
             snippet_path=COMPONENTS_SNIPPETS_DIR / f"{next_snip_no()}-scaffold.txt",
             update_snippets=update_snippets,
             snippet_replace_regex=[

examples/docs_snippets/docs_snippets_tests/snippet_checks/guides/components/test_components_docs_creating_a_component.py

@@ -36,7 +36,7 @@ def test_components_docs_index(
     with isolated_snippet_generation_environment() as get_next_snip_number:
         # Scaffold code location
         _run_command(
-            cmd="dg scaffold project my-component-library --use-editable-dagster && cd my-component-library",
+            cmd="dg scaffold project my-component-library --python-environment uv_managed --use-editable-dagster && cd my-component-library",
         )
 
         #########################################################