feat: display tokens per second for assistant messages

[edlsh]

Summary

Implements tokens/second display for assistant messages per #5374.

Changes

• Track firstToken timestamp when streaming begins
• Display tok/s rate next to response duration (only for completed text responses)
• Minimum 250ms elapsed time threshold to avoid noisy metrics
• Accumulate tokens instead of overwriting for multi-part responses
• Comprehensive test coverage for token utilities

Notes

If styling is not the direction the team wants to take, I'm happy to make revisions or move the display to a different location.

Closes #5374

[edlsh]