docs(models): document json_schema 400 on deepseek-v4-flash and tool-name 400

[sre-helmcode]

What

• Model card, deepseek-v4-flash (EN src/content/docs/models.mdx, ES src/content/docs-es/models.mdx): the description and capability list now state that response_format json_object is supported, json_schema is not and is rejected with a 400, and that schema-constrained output should use qwen3.6 or gemma4.
• src/data/openapi.json:
  • ResponseFormat.description: json_schema works on qwen3.6 and gemma4; on deepseek-v4-flash it is rejected with a 400 before the request reaches the model, and json_object still works there.
  • Tool.function.name: adds "pattern": "^[a-zA-Z0-9_-]{1,64}$" and says a name outside that pattern is rejected with a 400, on every model.
• src/lib/openapiSpec.test.ts: new documented 400s block pinning the pattern, the ResponseFormat sentence and both locale cards; the existing structured-output example check now reads the "works on" clause (the description now also names a model that rejects json_schema, so a plain mention was no longer enough).

Why

api.nan.builders is starting to reject both shapes with a 400 before routing. The spec already described the tool-name charset and where json_schema works, but the deepseek-v4-flash card did not say structured output via json_schema is unsupported. Members should find this in the docs, not from the error.

Evidence

• npm test: 66 files, 1309 tests passed.
• Mutation check: with the doc/spec changes stashed, 4 of the new/tightened tests fail; restored, all pass.
• npm run build: completes (only the existing chunk-size / punycode warnings); rendered model chunks contain the new copy in both locales.
• openapi.json parses; the existing $ref resolution and catalogue tests still pass.

Deploy notes

No VERSION file. .github/workflows/deploy.yml runs npm ci, npm test, npm run build and wrangler deploy to Cloudflare Workers on push to main, so merging deploys. No env vars or migrations.

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