chore(ai-gemini): remove retired model ids + cascade workspace consumer migration

[tombeckenham]

`packages/typescript/ai-gemini/src/model-meta.ts` carries model entries that no longer appear in Google's published list (https://ai.google.dev/gemini-api/docs/models). The most-recently-added gap (`gemini-3.5-flash`) was already addressed by #610. This issue covers the opposite side of the freshness audit: removing retired ids and migrating workspace consumers off them.

Authoritative sources

• Docs page: https://ai.google.dev/gemini-api/docs/models
• Live endpoint: `GET https://generativelanguage.googleapis.com/v1beta/models?key=\$GEMINI_API_KEY\`
• Google API Discovery doc: `https://generativelanguage.googleapis.com/\$discovery/rest?version=v1beta\` (Google's discovery format, references OpenAPI 3.0.3 for sub-schemas)

In this repo but not in Google's current docs (candidates for removal)

Repo constant	Repo id	Reason
`GEMINI_3_PRO`	`gemini-3-pro-preview`	Not in Google's docs anymore — appears retired in favor of `gemini-3.1-pro-preview` and `gemini-3.5-flash`
`GEMINI_2_5_FLASH_PREVIEW`	`gemini-2.5-flash-preview-09-2025`	Superseded by stable `gemini-2.5-flash`
`GEMINI_2_5_FLASH_LITE_PREVIEW`	`gemini-2.5-flash-lite-preview-09-2025`	Superseded by stable `gemini-2.5-flash-lite`
`GEMINI_2_FLASH`	`gemini-2.0-flash`	Entire 2.0 line retired from Google's published list
`GEMINI_2_FLASH_LITE`	`gemini-2.0-flash-lite`	Same
`GEMINI_2_FLASH_IMAGE`	`gemini-2.0-flash-preview-image-generation`	Same 2.0 retirement

Also worth adding: the stable `gemini-3.1-flash-lite` (current repo only has the `-preview` variant).

Cascading workspace consumers to migrate

Removing the retired ids breaks consumers that hardcode them. Each needs to be moved to a current id (e.g. `gemini-2.5-flash` as the default):

• `testing/e2e/src/lib/providers.ts` — `defaultModels.gemini = 'gemini-2.0-flash'`
• `testing/panel/src/lib/model-selection.ts` — `'gemini-2.0-flash'`, `'google/gemini-2.0-flash-001'`
• `testing/panel/src/routes/api.{chat,structured,summarize,image}.ts` — defaults to `gemini-2.0-flash`
• `testing/panel/tests/vendor-config.ts` — same
• `packages/typescript/ai-gemini/tests/chat-per-model-type-safety.test.ts` — extensively uses `gemini-2.0-flash` / `gemini-2.0-flash-lite` / `gemini-3-pro-preview`
• `packages/typescript/ai-gemini/tests/model-meta.test.ts` — same
• `packages/typescript/ai-gemini/tests/gemini-adapter.test.ts` — same
• `packages/typescript/ai-gemini/tests/tools-per-model-type-safety.test.ts` — `gemini-2.0-flash-lite`
• `examples/ts-react-chat/src/routes/generations.structured-output.tsx` — Gemini dropdown (added in Route structured-output through native combined-mode where supported (skip finalization round-trip) #605)
• Type maps: `GeminiChatModelProviderOptionsByName`, `GeminiChatModelToolCapabilitiesByName`, `GeminiModelInputModalitiesByName`, `GEMINI_MODELS`
• `GEMINI_COMBINED_TOOLS_AND_SCHEMA_MODELS` (added in Route structured-output through native combined-mode where supported (skip finalization round-trip) #605) — remove `gemini-3-pro-preview` reference

Out-of-scope-but-worth-checking-in-the-same-pass

Equivalent drift likely affects sibling categories:

• Image variants (`gemini-2.0-flash-preview-image-generation` retired; `imagen-4.0-*` variants vs Google's headline `imagen`)
• Live audio (`gemini-2.5-flash-native-audio-preview-12-2025` is newer than the commented-out 09-2025 variant)
• Veo video models (`VEO_3` / `VEO_3_FAST` / `VEO_2` not in current docs)
• Embedding (`gemini-embedding-2` is new; repo has no embedding adapter)

Scope this PR's call.

Acceptance

1. `ai-gemini/model-meta.ts` text/chat entries reflect Google's current published list — retired entries gone.
2. Stable `gemini-3.1-flash-lite` added alongside the existing preview variant.
3. All workspace consumers of removed ids migrated to current ids.
4. All affected tests still pass.
5. `GEMINI_COMBINED_TOOLS_AND_SCHEMA_MODELS` updated to drop retired ids.

Context

Surfaced while adding Gemini to the structured-output streaming demo in #605. Filed separately because (a) the routing work in #605 doesn't depend on which ids exist, just the set-membership check; (b) the cascading consumer migration is its own focused review; (c) `gemini-3.5-flash` (the most-needed addition) is already covered by #610.