fix(antigravity): add Gemini 3.5 and 3.6 Flash routes

[smallmain]

Summary

• expose Gemini 3.5, 3.6, and the current default Gemini 3.7 Flash model in the Antigravity catalog
• translate canonical Flash IDs and reasoning levels to the runtime IDs accepted by Antigravity
• preserve all known Gemini 3.5 wire IDs so repeated resolution and manually configured runtime IDs cannot be downgraded
• cover the final streamChat request model and thinkingConfig without requiring online credentials

Root cause

The Antigravity provider used a stale hard-coded model list, so the current Flash models did not appear. Requests for a manually configured bare gemini-3.6-flash ID were forwarded unchanged, but Antigravity accepts tier-specific runtime IDs, which caused the reported 404. The generic suffix parser also treated the valid Gemini 3.5 medium wire ID gemini-3.5-flash-low as the canonical low tier and downgraded it to the extra-low route.

Validation

• targeted resolver and streamChat Vitest coverage: 27 tests passed
• TypeScript source and test projects passed
• npm run test:unit -- --exclude test/unit/plan4-provider-catalog.test.ts: 104 files and 1245 tests passed
• the excluded DeepSeek FIM assertion fails identically on unmodified main and is unrelated to this change

Fixes #285