fix(#8136): expand pro-low fallback chain with gemini-pro-agent and pro-high

[rafaumeu]

Closes #8136

Problem

gemini-3.1-pro-low and gemini-3-pro-low both return HTTP 400 from the Antigravity upstream after a silent rename. The fallback chain for gemini-3.1-pro-low only had these two candidates, so the chain was exhausted immediately with no working alternative.

Fix

Expanded the gemini-3.1-pro-low fallback chain to include gemini-pro-agent (CLIProxyAPI's id) and gemini-3.1-pro-high as additional candidates:

```
gemini-3.1-pro-low → gemini-3-pro-low → gemini-pro-agent → gemini-3.1-pro-high
```

This gives the executor two more chances to hit a working upstream id before surfacing the 400 error.

Testing

• Updated the unit test asserting the pro-low chain shape
• All 8 existing chain behavioral tests pass (retries, exhausted-chain, happy path, non-pro isolation)

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.

[diegosouzapw]

Thanks for the fix — the gemini-3.1-pro-low fallback chain expansion (adding gemini-pro-agent and gemini-3.1-pro-high) is correct and well tested; I verified the release tip still only has the old 2-candidate chain, so this closes #8136 as intended.

However, this PR is based on main, which is currently far behind release/v3.8.49, and it bundles 7 other commits (#8074, #8072, #8059, #8081, #8056, #8141) that already have their own PRs open against release/v3.8.49 (#8165, #8062, #8162, #8058, #8143). Merging this branch as-is would revert already-merged work on the release branch:

• It reverts the Claude Antigravity contextLength fix (1,048,576 → 200,000 for claude-sonnet-5 / claude-opus-4-6-thinking / claude-sonnet-4-6), which landed on release via fix(models): update Anthropic model contextLength to 1M #7129 after this branch was cut.
• It deletes 5 behavioral tests in tests/unit/agy-pro-fallback-chain-3786.test.ts that already exist on release tip (exception-handling, AbortError propagation, mixed 400/throw paths) because this branch predates them.

We'll extract just the open-sse/config/antigravityModelAliases.ts pro-low chain change (plus the matching test assertion) onto release/v3.8.49 and merge that piece — the other 7 commits will be handled through their existing sibling PRs instead, so nothing is lost. No action needed on your end unless you'd like to re-open future fixes directly against release/v3.8.49 to avoid this kind of base-drift bundling.

[mergify]

⚠️ The sha of the head commit of this PR conflicts with #8113. Mergify cannot evaluate rules on this PR. Once #8113 is merged or closed, Mergify will resume processing this PR. ⚠️

[diegosouzapw]

Thanks, @rafaumeu! 🙏 Closing this one — after rebasing onto the current release tip, the expanded pro-low chain adds a gemini-3.1-pro-high fallback, but release/v3.8.49 (post-#8013) has a guard test (agy-pro-fallback-chain-3786.test.ts) asserting that rejected pro-high discovery ids have NO fallback chain, so this conflicts semantically (2 tests fail). The pro-low expansion with gemini-pro-agent is still valuable — a focused re-submit against the current tip that keeps pro-high out of the chain (or updates the guard with a rationale) would merge cleanly. 🙏