Refuse unknown model ids before reserving

[skyfallwastaken]

Requests for a model id OpenRouter doesn't list used to go upstream anyway. OpenRouter rejected them with a 400, but the gateway still reserved, recorded and settled each one, so every typo left a "phantom" error row in Activity.

Now the proxy refuses an id the model listing doesn't know with a 400, before anything is reserved or recorded. The message is the same one OpenRouter uses: <id> is not a valid model ID.

An id resolves if it matches, in order:

1. a listed id (this includes ~ aliases and listed variants such as :free and :batch);
2. a listed canonical_slug (dated ids);
3. the base id of a routing variant (openai/gpt-4o-mini:nitro, :floor, …). As a bonus, these now reserve at the base model's price instead of the flat unknown-model hold.

If the listing itself can't be fetched, requests still go through and OpenRouter decides, as before.

Screenshots

The same two typo'd requests from a fresh account, then that account's Activity:

# neo
POST /proxy/v1/chat/completions {"model":"openai/gpt-5-nanoo"}                → 400 (forwarded to OpenRouter)
POST /proxy/v1/chat/completions {"model":"notreal/this-model-does-not-exist"} → 400 (forwarded to OpenRouter)

# this branch
POST /proxy/v1/chat/completions {"model":"openai/gpt-5-nanoo"}                → 400 {"error":"openai/gpt-5-nanoo is not a valid model ID"}
POST /proxy/v1/chat/completions {"model":"notreal/this-model-does-not-exist"} → 400 {"error":"notreal/this-model-does-not-exist is not a valid model ID"}
POST /proxy/v1/chat/completions {"model":"openai/gpt-4o-mini:nitro"}          → 200

Before (neo): two phantom error rows	After: only the real request

(The :nitro row still says "nitro" on this branch. The Activity PR fixes that display.)

Testing

• proxy.integration.test.ts:
  • An unlisted model gets a 400, makes no upstream call, and leaves no billing rows.
  • A :nitro variant is forwarded unchanged, finalized, and reserved at the base model's price.
• The proxy.test.ts fixture now lists the requested model with no fixed price, so the unknown-model-hold tests still exercise the hold.
• bun run typecheck: 0 errors. bun test: 0 fail.

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