fix(session): warn on cache busts

[zcxGGmu]

Issue for this PR

Closes#40790

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds a small diagnostic for likely Anthropic prompt-cache busts. When a completed step reports a large cache write with zero cache reads for an Anthropic/Claude model, the session processor now emits a warning with the session, model, finish reason, cache-write tokens, cache-read tokens, and cost.

This does not revive the previously rejected object-identity caching approach from #36852. It only makes the high-cost pattern reported in #40790 visible in logs so users and maintainers can confirm when it happens.

How did you verify your code works?

• RED: bun test test/session/compaction.test.ts --test-name-pattern "flags likely Anthropic cache busts|does not flag ordinary cache hits" --timeout 30000 failed because SessionNs.cacheBustWarning did not exist.
• GREEN: bun test test/session/compaction.test.ts --test-name-pattern "SessionNs.getUsage" --timeout 45000 — 17 pass.
• bun run typecheck
• bun run lint packages/opencode/src/session/session.ts packages/opencode/src/session/processor.ts packages/opencode/test/session/compaction.test.ts
• git diff --check

Notes: two broader session tests fail the same way on a clean dev baseline in this Windows checkout:

• stops quickly when aborted during retry backoff
• retry OpenAI-compatible midstream server errors

Screenshots / recordings

Not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

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1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

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