feat(openrouter): send HTTP-Referer and X-Title headers to identify app (#11857)

* feat(openrouter): send HTTP-Referer and X-Title headers to identify app

Adds `HTTP-Referer: https://www.continue.dev/` and `X-Title: Continue`
headers to all OpenRouter API requests, as recommended by the OpenRouter
docs. This lets OpenRouter attribute requests to Continue in their
dashboard and App Showcase.

Headers are injected via both `requestOptions.headers` (for SDK-based
requests) and `getHeaders()` (for direct fetch calls). User-configured
headers take precedence.

Closes #4049

* refactor: remove redundant getHeaders() override

The OPENROUTER_HEADERS are already injected via requestOptions.headers
in the constructor. Since fetchwithRequestOptions merges
requestOptions.headers into all requests (including direct fetch calls
like fimStream and rerank), the getHeaders() override was redundant.