fix(kosong): propagate message serialization options

[lihailong00]

Summary

• propagate Pydantic serialization options through Message.content
• omit nested media id: null when providers call model_dump(exclude_none=True)
• preserve default dumps, non-null IDs, and the single-text-part compatibility representation

Reproduction and verification

Before the fix, media parts and real OpenAI Legacy requests still contained nested id: null. After the fix:

• message and provider snapshot tests — 43 passed
• targeted Ruff and Pyright — passed

Closes #1791

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.