fix(openrouter): pass attribution headers via httpx `default_headers` (#36347)

Fixes #36339

---

The `openrouter` SDK v0.8.0 renamed `x_title` to `x_open_router_title`,
breaking `ChatOpenRouter` instantiation with the default `app_title`.

Rather than chasing SDK parameter renames across versions, all three
attribution headers are now injected via httpx `default_headers` —
version-agnostic and consistent with how `app_categories` was already
handled.

## Changes
- Pass `HTTP-Referer`, `X-Title`, and `X-OpenRouter-Categories` as httpx
client default headers in `_build_client` instead of SDK constructor
kwargs (`http_referer`, `x_title`), making the integration compatible
across `openrouter>=0.7.11,<1.0.0`
- Move `_build_client()` inside the `try/except ImportError` in
`validate_environment` so a version-mismatch `ImportError` from
`openrouter.utils` gets the friendly install message instead of a raw
traceback
- Add `warnings.warn` in `_wrap_messages_for_sdk` for two previously
silent fallbacks: failed `openrouter.components` import (file blocks
sent as raw dicts) and unknown message roles passed through to the API
- Clarify `max_retries` docstring to explain the ~150s-per-unit backoff
mapping; drop stale `(v0.6.0)` version reference in
`_wrap_messages_for_sdk`