fix, FilePart, and _parsePart;

[huanguan1978]

FilePart Api ref: https://ai.google.dev/api/caching#FileData

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[huanguan1978]

finish review

[huanguan1978]

finish review

[iqbalbhatti49]

• The new implementation introduces support for inlineData and fileData content parts. Great work in enhancing functionality!
• Proper handling of inlineData with DataPart and decoding using base64Decode is a smart addition.
• FilePart implementation includes clear handling of mimeType and fileUri, with serialization via toJson()—nicely structured!
• Adding executableCode support provides a foundation for future extensibility.

Overall, this update significantly improves the code's capabilities—excellent work!