fix(pi): strip trailing assistant messages, improve sanitize and error guards

[iceteaSA]

Improvements to pi/convert.ts beyond what was merged in #17:

• Strip trailing assistant messages from converted requests to prevent Anthropic prefill 400 errors on some models
• Unicode regex (/gu flag) for spec-compliant surrogate pair replacement
• Simplified sanitizeToolId with cleaner length check (removes redundant guard)
• Improved error content guard catches empty string case (content === '') that upstream misses
• Comprehensive test coverage for all conversion logic (21 tests)

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Summary by cubic

Prevents Anthropic prefill 400s by stripping trailing assistant messages in packages/pi conversions. Also improves sanitization and error handling, plus adds full tests and a test script.

• Bug Fixes
  • Strip trailing assistant messages from converted requests to avoid prefill errors on some Anthropic models.
  • Use Unicode /gu flag in surrogate replacement for spec compliance.
  • Simplify sanitizeToolId with deterministic allowed chars and a clean length check.
  • Guard tool_result with is_error=true and empty content (including empty string) by inserting "Error".
  • Add test script (bun test src/tests) and 21 tests covering conversion logic.

^{Written for commit b659ab4. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

1 issue found across 3 files

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