Add validateAnthropicTurns() to merge consecutive user messages that can
occur when steering messages are injected during streaming. This prevents
the API from rejecting requests due to improper role alternation.
Changes:
- Add validateAnthropicTurns() function in pi-embedded-helpers.ts
- Integrate validation into sanitization pipeline in pi-embedded-runner.ts
- Add user-friendly error message for role ordering errors
- Add comprehensive tests for the new validation function
