[RaphaelFakhri]

Summary

DeepgramClient and AsyncDeepgramClient add x-deepgram-session-id directly to the headers dict that the caller passes in. The generated client wrapper stores a reference to that dict, so clients built from one shared dict all send the session ID of the last client created, and the caller's dict changes.

This change copies headers before the session header is added, in both clients.

Changes

• src/deepgram/client.py: copy the caller's headers into a new dict before setting x-deepgram-session-id.
• tests/custom/test_client_construction_and_helpers.py: add a test that builds two clients from one shared dict and checks that each client sends its own session_id and that the shared dict is unchanged.

Testing

• New test fails on main (2 failed: the first client sends the second client's session ID) and passes with the fix (2 passed).
• pytest tests/custom tests/utils: 1081 passed, 1 skipped.
• ruff check and mypy pass on the changed files.

Fixes #824

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