fix(gemini): update model catalog for retired/new Gemini models (#11793)

* fix(gemini): update model catalog for retired/new Gemini models

Gemini 1.x models are fully retired (returning 404), gemini-3-pro-preview
was shut down March 9 2026, and new 3.1 series models were missing.

- Add gemini-3.1-pro-preview, gemini-3.1-flash-lite-preview, gemini-3.1-flash-image-preview
- Replace gemini-3-pro-preview with gemini-3.1-pro-preview everywhere
- Remove retired Gemini 1.x models (gemini-1.0-pro, gemini-1.5-pro, gemini-1.5-flash, gemini-1.5-flash-8b)
- Remove retired experimental 2.0 image gen models
- Fix default model from retired gemini-pro to gemini-2.5-flash
- Update docs, onboarding config, VSCode schema, GUI model picker, VertexAI provider
- Update isRecommendedAgentModel regex and tests

Fixes #10996, #10998

* fix: address review comments

- Add missing gemini-3.1-flash-image-preview to VS Code schema enum
- Tighten isRecommendedAgentModel regex to match 3-flash-preview (not bare 3-flash)
- Update case-insensitivity tests for new model names

* fix: fix misleading test description and inconsistent doc heading

- Rename test to "Gemini 3.1 Pro and 3 Flash Preview models" to match assertions
- Update chat.mdx heading from "Gemini 3 Pro" to "Gemini 3.1 Pro"