fix: preserve OpenRouter request parameters

[ElliotEtag]

Why

Pollinations sends OpenRouter traffic through the generic OpenAI custom-host adapter. It preserves fields but drops OpenRouter's provider routing object. The native adapter preserves routing but drops fields such as stop. Live probes reproduced both failures: the generic adapter ignored an impossible provider.only allowlist; the native adapter enforced it but ignored stop.

Production evidence

Tinybird production, 2026-08-21:

• Seven-day all-zero usage: gemini-fast 3,734/645,788 successes (142 fully missing), gemini 388/3,406 (166 missing), and gemini-flash-lite-3.5 43/4,250.
• The rolling-hour symptom was limited to those three pinned Gemini routes; non-Gemini OpenRouter routes had only isolated seven-day events: Mistral 12, Llama Scout 2, Mercury 2.

All three should pin Vertex, but the generic adapter removes that pin. Gemini Fast has most volume; the symptom is Gemini-family-wide.

Fix and long-term result

• Preserve the OpenRouter Chat Completions contract, including routing, stop, usage, tools, and streaming options.
• Cover the complete parameter map with regression tests.

This establishes one provider-aware adapter. Pollinations can then switch every OpenRouter route at the shared factory without per-model patches or post-request recovery. This PR does not deploy or switch traffic.

Checks

• Focused tests, build, and local startup pass; repository-wide checks retain unrelated main failures.