fix(voice): tolerate formatting changes in preemptive transcripts

[swayamg20]

Summary

• reuse preemptive generations when the final transcript differs only in casing, whitespace, or punctuation
• reconcile the speculative user message with the finalized transcript and on_user_turn_completed edits before scheduling it
• continue invalidating speculative generations when the recognized words or request context change
• clarify the invalidation warning

Fixes #5766

Testing

• uv run pytest tests/test_agent_session.py --unit -q -k preemptive_generation — 5 passed
• uv run pytest --plugin soniox -q — 28 passed
• make check

[theomonnom]

lgtm. I'd remove the test file tho, it doesn't add much value, and it's a bit fragile since it relies on the logger.

[swayamg20]

Thanks @theomonnom , sure, sounds right. Removing the test file.

[theomonnom]

Thanks!