fix(antigravity): align User-Agent token order and auth_method with 2.5.5 decompilation

[dbc-hbin]

Summary

Align token order and components in antigravityUserAgent with the decompiled Antigravity 2.5.5 Go Language Server (codeassistclient.(*CodeAssistClient).setHeaders @ 0x1018fbe00):

• Decompiled token sequence: os_type=%s → arch=%s → aidev_client → auth_method=%s (joined with ; ).
• Enclosing format: antigravity/ide/%s (%s) using subclient (ide) and version (2.5.5).
• Resulting User-Agent: antigravity/ide/2.5.5 (os_type=windows; arch=amd64; aidev_client; auth_method=oauth).
• Wire adapter: Use antigravityUserAgent() directly for ANTIGRAVITY_REQUEST_UA to ensure shared trimmed override handling.
• Configurability: Parameterized authMethod on antigravityUserAgent(version, authMethod = "oauth").

Evidence & Verification

• Decompiled binary: language_server_macos_arm 126MB Go1.26.5 (setHeaders @ 0x1018fbe00).
• Live Wire Test with Active OAuth Token (daily-cloudcode-pa.googleapis.com):
  • fetchAvailableModels: Returns all 28 models including gemini-3.7-flash-{low,medium,high} (200 OK).
  • generateContent (gemini-3.7-flash-low): Returns 200 OK with streaming completion.
• Local Tests:
  • bun test tests/client-fingerprint.test.ts tests/google-antigravity-wire.test.ts tests/google-antigravity-oauth.test.ts (75 passed, 0 failed)
  • bun run typecheck (0 errors)

Review readiness checklist

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• All CI tests are green on my local testing.
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• I resolved all correct Codex and CodeRabbit findings.
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Summary by CodeRabbit

• Improvements
  • Updated Antigravity request identification to include operating system, architecture, client, and authentication method details in a consistent order.
  • Added support for customized user-agent values, including whitespace trimming and fallback to the default when no valid override is provided.
  • Standardized Google Antigravity requests to use the updated user-agent format.