feat: expose request_id on ImageResponse

[aheze]

Summary

• ImageResponse (sync and aio) now exposes a request_id property, populated from the x-request-id header on the gRPC response's initial metadata of image.sample() / image.sample_batch(). Returns None if the header is absent.
• This is the id xAI can look up in server logs, so API customers can include it when reporting a failed or moderated generation. Today the SDK discards the gRPC call object, so the header is dropped and customers have no way to reference their request.
• Sync uses GenerateImage.with_call(...), aio awaits call.initial_metadata(); a shared _extract_request_id helper in image.py handles both metadata formats.

Test plan

• Test gRPC server now sends x-request-id initial metadata on GenerateImage; new sync + aio tests assert response.request_id round-trips for sample and sample_batch, plus a default-None unit test.
• Full suite: 769 passed. ruff check, ruff format --check, and pyright clean on changed files.

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