fix(schema): preserve Extra fields when merging message parts during stream concatenation

[JonXSnow]

• mergeTextParts: collect and merge Extra from all MessageOutputPart chunks
• mergeReasoningParts: collect and merge Extra from all MessageOutputPart chunks
• mergeAudioParts: merge outer MessageOutputPart.Extra in addition to inner Audio.Extra
• mergeTextPartsInChunk: collect and merge Extra from ToolOutputPart text chunks in ConcatToolResults
• Add test cases covering Extra merging for text, reasoning, audio (outer), and tool output parts

[codecov]

Codecov Report

❌ Patch coverage is 82.75862% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.58%. Comparing base (04d2ae0) to head (d15e756).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
schema/message.go	82.75%	9 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #855      +/-   ##
==========================================
+ Coverage   81.56%   81.58%   +0.02%     
==========================================
  Files         146      146              
  Lines       15932    15994      +62     
==========================================
+ Hits        12995    13049      +54     
- Misses       1995     1997       +2     
- Partials      942      948       +6     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.