Additional HTTP headers to include on every request to OpenRouter.
Headers set here become the underlying httpx client's default headers, so
they are sent on every request to OpenRouter. Useful for upstream provider
features that require custom headers — for example, xAI's x-grok-conv-id
for sticky-routing prompt cache hits, region routing, A/B test bucketing
headers, or provider-specific authentication. Whether a given header is
forwarded to the upstream provider (versus consumed by OpenRouter itself)
is determined by OpenRouter; consult its docs for which headers propagate.
Because these headers may contain credentials, they are excluded from LangChain serialization.
Example: {"x-grok-conv-id": "session-abc123"}
Headers set via this field are merged with the OpenRouter app-attribution
headers (HTTP-Referer, X-Title, X-OpenRouter-Categories); on a
collision the value from default_headers takes precedence, matched
case-insensitively (HTTP header names are case-insensitive).
