Save OpenRouter generation_id in LLM edit history

[dadukhankevin]

Summary

• callLLM() now returns the OpenRouter generation_id from the API response alongside the completion content
• The generationId is stored in EditHistory metadata when LLM completions are saved to .codex cells
• All existing callers updated to handle the new return type

Closes #753

Test plan

• Trigger an LLM completion on a cell and verify the resulting edit in .codex file metadata contains a generationId field
• Verify A/B test variant selection still works correctly
• Verify chat/smart edits still function (they use separate callLLM paths)

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