fix: use AI SDK totalUsage instead of local estimate for token usage display

Since ai@5.0, fullStream finish events no longer carry usage data.
The code was falling back to local tiktoken estimates which are much
larger than actual usage. Now reads result.totalUsage after stream
ends and yields it as a custom 'usage' chunk.

Also removes noisy chatStore session update debug logs.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>