test(fireworks): cover request-level extra headers

[Mason Daugherty (mdrxy)]

Fireworks chat users can pass request-specific headers to the SDK, but the integration did not have targeted coverage or examples for session-affinity and multi-turn headers. This adds explicit coverage for sync, async, and streaming calls, and documents the supported invocation patterns.

Changes

• Documented ChatFireworks request-level extra_headers examples for session affinity and multi-turn sessions, plus the SDK-level prompt_cache_key alternative.
• Added TestExtraHeaders coverage showing extra_headers reach top-level SDK kwargs for sync and streaming calls rather than being folded into extra_body.
• Covered the async ainvoke path so request-specific headers are verified across the main call modes.

[open-swe]

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