feat(schema): wire OpenAI ReasoningExtension into Reasoning block

[mrh997]

Summary

Adds an OpenAI-specific reasoning extension to the schema and carries it through reasoning concatenation, so OpenAI reasoning content (and its per-content index) survives stream aggregation alongside the existing Text and Signature fields.

API Changes

1. Added: openai.ReasoningExtension and openai.ReasoningContent types. ReasoningContent holds the reasoning text and an optional Index *int marking its position in the final response.
2. Added: openai.ConcatReasoningExtensions, which merges streamed chunks by index — same-index chunks are concatenated, results are ordered by index, and a missing index returns an error.
3. Added: Reasoning.OpenAIExtension *openai.ReasoningExtension field on the schema Reasoning block.

Key Changes

1. concatReasoning now collects each chunk's OpenAIExtension and folds them via ConcatReasoningExtensions, propagating any concat error. It also skips nil reasoning entries during aggregation.

[codecov]

Codecov Report

❌ Patch coverage is 82.60870% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.09%. Comparing base (30f5961) to head (38eae2f).

Files with missing lines	Patch %	Lines
schema/agentic_message.go	27.27%	5 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1063      +/-   ##
==========================================
- Coverage   83.14%   83.09%   -0.05%     
==========================================
  Files         162      162              
  Lines       23060    23104      +44     
==========================================
+ Hits        19173    19199      +26     
- Misses       2632     2641       +9     
- Partials     1255     1264       +9     

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[shentongmartin]

LGTM. The previous concern around ReasoningContent.Index has been clarified as internal streaming merge behavior, and the runtime blocker has been addressed.