[Models] /v1 gateway: input-validation hardening — 400-vs-500 envelope consistency, json_schema nesting, 'developer' role

[heskew]

Follow-ups surfaced by cross-model review during #1856 (all pre-existing in the #1616 gateway code, none introduced there — filed separately so the test PR stays scoped):

1. Malformed input shapes surface as 500s instead of OpenAI-shape 400s. Unguarded paths throw plain TypeError/SyntaxError (no statusCode), which serialize as 500s; OpenAI SDK clients retry 5xx, so these should be invalid_request_error 400s:
   • non-array tools (req.tools?.length is truthy for a string; translateTools's .map throws) — chatCompletions.ts / translation.ts
   • tools[] element without function, and messages[] element that is null — translation.ts
   • malformed JSON body: body = await body rejects outside the try/catch — embeddings.ts, chatCompletions.ts
2. response_format: json_schema nesting: toGenerateOpts forwards the OpenAI wrapper { name, schema, strict } as the schema; the actual schema is nested at json_schema.schema. Verify what the backend contract expects and unwrap accordingly — translation.ts.
3. role: 'developer' unmapped: newer OpenAI models send role: 'developer'; translateMessages casts it through without mapping. Decide mapping (likely → system).

Context: #631 (Phase 4 of #510), gateway code lands via #1616.

🤖 Filed with Claude Code on behalf of @heskew from cross-model review findings