ci: update Swift 6.4 coverage workflow

[qiweiii]

Summary

• Fix Swift 6.4 Linux coverage artifact discovery.
• Support both .xctest and .so test bundles.
• Make coverage fail when package tests fail.
• Handle the unwritable-directory test on root CI runners.
• Use the OpenAI-compatible Polka Codes reviewer.
• Skip Polka review automatically when its credentials are unavailable.

Validation

• Code Coverage passed in CI.
• Project coverage passed at 67.71%.
• The focused Boka filesystem test passed locally.
• Codecov patch coverage reports one expected uncovered root-only test line.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 67.70%. Comparing base (87c29c1) to head (564c438).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
Boka/Tests/BokaTests/BokaTests.swift	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #400      +/-   ##
==========================================
+ Coverage   59.45%   67.70%   +8.24%     
==========================================
  Files         448      474      +26     
  Lines       38794    47160    +8366     
  Branches        0     1474    +1474     
==========================================
+ Hits        23065    31928    +8863     
+ Misses      15729    14585    -1144     
- Partials        0      647     +647     

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