feat: upgrade generative model to gemini-3-flash-preview

[NamanGoyalK]

Closes #3555

Updated the _geminiModel constant to models/gemini-3-flash-preview as requested.

Verification

• Verified locally that dart-services starts correctly.
• Tested against the /api/v3/suggestFix endpoint and confirmed valid 200 OK responses.
• Confirmed models/gemini-3-flash-preview is the required endpoint for the current API release (Jan 2026).

---

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

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[NamanGoyalK]

I am closing this pr and have opened an identical pr #3567 as this has some branching issues.