feat(llmloop): backfill comment thinking from turn output

[yingjiexu2002]

Description

Comments parsed from code_comment tool calls have a runtime-only thinking field that most models never populate (it is deliberately not advertised in the tool schema). This PR backfills it from the model's output of the current turn, so --format json output carries the review reasoning:

• Add ChatResponse.ReasoningContent() to expose the native reasoning content of a response (OpenAI reasoning_content in both streaming and non-streaming paths; Anthropic thinking blocks).
• In the agent loop, compute the turn's thinking text as reasoning content, falling back to the assistant message for models that do not expose reasoning, and backfill it into each collected comment whose thinking is empty. Explicit model-provided thinking always wins.
• Terminal output is unchanged (renderComment does not render thinking); only the JSON output gains the field.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactoring (no functional changes)
• Documentation update
• CI / Build / Tooling

How Has This Been Tested?

• make test passes locally
• Manual testing (describe below)

Added unit tests: backfill uses the turn's thinking text when a comment has no explicit thinking; explicit thinking is preserved; no backfill when the turn has no thinking text. Also verified end-to-end with a local review run (--format json) against providers with and without native reasoning output.

Checklist

• My code follows the project's coding style (go fmt, go vet)
• I have performed a self-review of my code
• I have added tests that prove my fix is effective or my feature works
• New and existing unit tests pass locally with my changes
• I have updated the documentation accordingly (if applicable)
• I have signed the CLA

Related Issues

N/A

[github-actions]

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