fix(auth): Revert to default credential

[Prajna1999]

Issue #1060

Summary

• providers.py: google provider now requires only api_key — dropped project_id, location, sa_key, gcs_bucket from required fields, and sa_key from sensitive fields.
• mappers.py: removed the guard that rejected text completions on the google provider; google + text now maps to a google-native config and executes via the api-key Gemini client (_execute_text).
• test_mappers.py: updated the stale rejection test to assert google + text now maps to google-native.

Why it's safe

• Credential change is strictly more permissive — no existing creds break; full-cred storage still works.
• The wired GoogleAIProvider needs only api_key; the Vertex provider (google_ai.py) isn't in the registry and has settings fallbacks for every dropped field.
• Only one test depended on the old behavior; now fixed.

Reviewer confirm

• ⚠️ sa_key removed from sensitive fields → any existing stored sa_key is now returned unmasked in GET /credentials. No test covers this. Intended?
• Suite not run live — test DB schema is stale locally.

Checklist

Before submitting a pull request, please ensure that you mark these task.

• Ran fastapi run --reload app/main.py or docker compose up in the repository root and test.
• If you've fixed a bug or added code that is tested and has test cases.

Notes

This was a hotfix already deployed to prod. To be merged with main for staging and next steps.

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[github-actions]

OpenAPI changes   ⚪ No API surface changes

Note

This PR does not modify the API contract.

_{main ↔ 624d5a71 · generated by oasdiff}

[Prajna1999]

Closing as duplicate to #1061 . The test cases have been taken care by it.