chore(ts-react-chat): sync structured-output Gemini dropdown to current model list

[tombeckenham]

The Gemini provider dropdown in `examples/ts-react-chat/src/routes/generations.structured-output.tsx` (added in #605) has two known mismatches against Google's current published model list (https://ai.google.dev/gemini-api/docs/models).

This is a narrowly scoped subset of #620 — useful for a maintainer who wants to fix the demo without taking on the full model-meta refresh.

Current dropdown state

```ts
gemini: [
{ value: 'gemini-3.5-flash', label: 'Gemini 3.5 Flash' }, // ✓ stable, in spec
{ value: 'gemini-3-pro-preview', label: 'Gemini 3 Pro (Preview)' }, // ✗ not in Google's docs anymore
{ value: 'gemini-3-flash-preview', label: 'Gemini 3 Flash (Preview)' }, // ✓
{ value: 'gemini-3.1-pro-preview', label: 'Gemini 3.1 Pro (Preview)' }, // ✓
{ value: 'gemini-3.1-flash-lite-preview', label: '...' }, // ✓
// missing: 'gemini-3.1-flash-lite' (stable GA)
]
```

Fix 1: remove `gemini-3-pro-preview`

Google's docs no longer list this id — appears retired in favor of `gemini-3.1-pro-preview` and `gemini-3.5-flash`. Safe to remove from the dropdown alone: `GEMINI_3_PRO` is still in `ai-gemini/model-meta` (kept by #620 to keep type-map and consumers compiling), so removing it from just the dropdown doesn't cascade.

If a maintainer also wants to remove the constant from model-meta, that's #620 territory.

Fix 2: add `gemini-3.1-flash-lite` (stable)

Google's docs list both `gemini-3.1-flash-lite` (stable) and `gemini-3.1-flash-lite-preview` (preview) as separate routes. The repo only has the preview entry.

This one requires touching `ai-gemini/model-meta` first — `geminiText('gemini-3.1-flash-lite')` would TypeScript-fail today because the parameter is typed as `(typeof GEMINI_MODELS)[number]` and the stable id isn't in that union. Minimum work:

1. Add `GEMINI_3_1_FLASH_LITE_STABLE` (or similar name) const in `ai-gemini/model-meta.ts` with `name: 'gemini-3.1-flash-lite'`.
2. Add `.name` to `GEMINI_MODELS` array.
3. Add to `GeminiChatModelProviderOptionsByName`, `GeminiChatModelToolCapabilitiesByName`, `GeminiModelInputModalitiesByName` type maps.
4. Add to `GEMINI_COMBINED_TOOLS_AND_SCHEMA_MODELS` set (it's a Gemini 3.x model, so qualifies for the Route structured-output through native combined-mode where supported (skip finalization round-trip) #605 native combined path).
5. Add to the dropdown.

Acceptance

• Dropdown lists every Gemini text/chat id from Google's current docs that the adapter supports.
• No dropdown entries that 404 against Google's API.
• `pnpm --filter @tanstack/ai-gemini test:types` clean.
• The structured-output demo (`/generations/structured-output`) round-trips for every dropdown option.

Context

Filed during #605 review. The dropdown drift was noticed but explicitly scoped out of #605 to keep the routing PR focused. #620 covers this plus the broader model-meta freshness work; this issue is the demo-only slice for anyone wanting a quick win.