feat(fireworks,groq,openrouter): add standard model property

[Mason Daugherty (mdrxy)]

Add a model property to ChatFireworks, ChatGroq, and ChatOpenRouter that returns model_name. These partners use Pydantic's Field(alias="model") on model_name, which means instance.model doesn't work as a read accessor after construction — it raises an AttributeError or returns the field descriptor. ChatOpenAI already has this property; this brings the remaining in-repo partners to parity.

[codspeed-hq]

Merging this PR will not alter performance

✅ 3 untouched benchmarks
⏩ 33 skipped benchmarks¹

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_{Comparing mdrxy/std-model-prop (294b659) with master (6b37ad4)}

Footnotes

1. 33 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩