feat(site): add claude-fable-5 and claude-mythos-5 known model defaults - #27131

Adds claude-fable-5 and claude-mythos-5 to the frontend Known Models catalog so the Chat Model admin panel can autofill defaults (context limit, max output tokens, reasoning effort, and pricing) for both models.

Changes

• Add claude-fable-5 and claude-mythos-5 entries to anthropicKnownModels, ordered ahead of the existing Opus/Sonnet/Haiku entries. Both carry a 1M context limit, 128k max output tokens, reasoningEffort: "high" (adaptive thinking path), and $10/$50 per million input/output token pricing with $1/$12.50 cache read/write.
• Update the header comment listing which Anthropic models use adaptive thinking.
• Extend anthropic.test.ts expectations for catalog order and reasoning defaults.

Data sourcing

claude-fable-5 values are taken from models.dev (retrieved 2026-07-09, last updated 2026-06-09). models.dev does not list claude-mythos-5 (limited release via Project Glasswing), but Anthropic documents that it shares Claude Fable 5's specs and pricing, so its entry mirrors the fable-5 values. This caveat is documented as a code comment on the entry.

Validation

• pnpm test src/pages/AgentsPage/components/ChatModelAdminPanel/knownModels: 45/45 tests pass
• biome check on the changed directory: clean
• tsc --noEmit on site/: clean

Generated with mux • Model: anthropic:claude-fable-5 • Thinking: xhigh