#22337: fix(models): add fallback Gemini 3.1 Pro Preview catalog entry

## What changed - Add a built-in fallback entry for Google provider model google/gemini-3.1-pro-preview in `src/agents/model-catalog.ts` when it is missing from discovered models. - Keep fallback behavior aligned for runtime usage: reasoning enabled, context window set to 1,000,000, and text/image input support. - Add tests in `src/agents/model-catalog.test.ts` covering: - model injection when missing, - no-duplicate behavior when already present. ## Why this fixes the issue Issue 22323 is caused by discovery missing the new Gemini 3.1 Pro Preview model, which can hide it from built-in provider selection. This fallback guarantees the catalog always contains that model and does not alter discovery semantics when a matching entry already exists. ## Tests run - `pnpm vitest run --config vitest.unit.config.ts src/agents/model-catalog.test.ts` - `pnpm test -- src/agents/model-catalog.test.ts` ## Edge cases - Existing entries for google/gemini-3.1-pro-preview are preserved and not duplicated. - Existing cache-reset/retry behavior and partial discovery failure handling remain unchanged. ## Infra flake / blockers - None known. <!-- greptile_comment --> <h3>Greptile Summary</h3> Adds fallback catalog entry for `google/gemini-3.1-pro-preview` model when it's missing from SDK discovery, following the same pattern as the existing OpenAI Codex Spark fallback. **Key changes:** - Implements `applyGoogle31ProPreviewFallback()` function with deduplication check - Configures model with `reasoning: true`, 1M context window, and text+image input support - Adds comprehensive test coverage for both insertion and deduplication scenarios - Updates existing test to use `toContainEqual` instead of strict equality for better flexibility <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk - The implementation follows an established pattern (OpenAI Codex fallback), includes proper deduplication logic, and has comprehensive test coverage validating both insertion and non-duplication scenarios. The change is isolated, well-tested, and addresses a specific discoverability issue. - No files require special attention _{Last reviewed commit: c903811} <!-- greptile_other_comments_section --> <!-- /greptile_comment -->