fix: ensure canonicalized paths remain accessible via strong references

[Boshen]

Summary

• Fixes Canonicalized path was dropped error #732 - "Canonicalized path was dropped" error that could occur in CI environments
• Ensures canonicalized paths have at least one strong reference in the cache to prevent premature dropping
• Adds comprehensive tests to prevent regression

Problem

The resolver was experiencing "Canonicalized path was dropped" errors due to weak references being used without ensuring a strong reference existed. When canonicalize_impl stored only weak references to canonicalized paths, if those paths weren't already in the cache, the Arc would be immediately dropped, causing the weak reference to fail on upgrade.

Solution

Modified canonicalize_impl in src/cache/cache_impl.rs to ensure canonicalized paths are added to the cache (if not already present) before downgrading to weak references. This maintains at least one strong reference while preserving the weak reference pattern needed to avoid memory leaks from circular references.

Test Plan

• Added test_canonicalized_path_not_dropped to verify canonicalization works correctly
• Added test_canonicalized_path_weak_reference to ensure the error doesn't occur
• All existing tests pass, including the memory leak test
• cargo test passes
• cargo clippy passes
• cargo check passes

🤖 Generated with Claude Code

[graphite-app]

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[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.58%. Comparing base (cebe9fc) to head (9e971fa).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/cache/cache_impl.rs	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #733      +/-   ##
==========================================
- Coverage   94.64%   94.58%   -0.06%     
==========================================
  Files          15       15              
  Lines        2914     2920       +6     
==========================================
+ Hits         2758     2762       +4     
- Misses        156      158       +2     

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[Boshen]

Merge activity

• Sep 28, 8:17 AM UTC: The merge label 'merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• Sep 28, 8:18 AM UTC: Boshen added this pull request to the Graphite merge queue.
• Sep 28, 8:20 AM UTC: The Graphite merge queue couldn't merge this PR because it failed for an unknown reason.

[codspeed-hq]

CodSpeed Performance Report

Merging #733 will not alter performance

_{Comparing fix-canonicalized-path-dropped (9e971fa) with main (7c29092)}

Summary

✅ 7 untouched
⏩ 2 skipped¹

Footnotes

1. 2 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩