fix(usage): retain reported usage on canceled or failed streams

[rexdotsh]

Summary

• Record provider-reported token usage once when a Claude or Codex SSE stream is canceled or fails, not only at a clean EOF.
• Claude message_start already reports input/cache tokens; preserve those even when a turn ends before message_delta. Codex can preserve any usage already observed in a parsed event before cancellation.
• Cover Claude cancellation and upstream failure, plus Codex cancellation after a usage-bearing event.

Limit: this cannot recover tokens the upstream has not reported yet. In particular, Codex commonly sends usage only in its terminal event, so a cancellation before that event can still be unmeasured. No estimates are fabricated.

Addresses the recoverable part of audit finding #7 only, independently of #49 and the other audit PRs. bun test (143 passing), bun lint, bun typecheck. No live provider requests. Leave unmerged for review.