fix: don't drop canonicalized path by cache clear

[sapphi-red]

Canonicalized path was dropped error started to happen when I started to call resolver.cache_clear() frequently (so that I can clear the cache for each resolution to support vitest-dev/vitest#8754 (comment)).

My guess of the cause is:

• resolver.cache_clear() is called while a resolution is happening
• self.paths is cleared because of that call
• this Arc::downgrade removes the last strong reference:

  oxc-resolver/src/cache/cache_impl.rs

  Line 283 in 9de3088

  res.map(|cp| Arc::downgrade(&cp.0))

• this weak.upgrade() errors:

  oxc-resolver/src/cache/cache_impl.rs

  Line 288 in 9de3088

  weak.upgrade().map(CachedPath).ok_or_else(|| {

This PR solves that by using Mutex<Weak<_>>> instead of OnceLock<Weak<_>>. Since weak.upgrade() may return None, I changed the code to resolve when weak returned None. The previous code assumed that weak.upgrade() always returns a value and that allowed us to use OnceLock.

I guess this will have a negative impact on perf...

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[codecov]

Codecov Report

❌ Patch coverage is 82.75862% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.70%. Comparing base (9de3088) to head (3fbd440).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/cache/cache_impl.rs	82.14%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #791      +/-   ##
==========================================
+ Coverage   93.59%   93.70%   +0.10%     
==========================================
  Files          17       17              
  Lines        3093     3081      -12     
==========================================
- Hits         2895     2887       -8     
+ Misses        198      194       -4     

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[codspeed-hq]

CodSpeed Performance Report

Merging #791 will not alter performance

_{Comparing fix/dont-drop-canonicalized-path-by-cache-clear (3fbd440) with main (9de3088)}

Summary

✅ 10 untouched
⏩ 5 skipped¹

Footnotes

1. 5 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. ↩

[Boshen]

This PR has not been forgotten, I'm still seeking a better solution.

[Boshen]

What I have found so far after fighting with Claude:

... Told Claude to add tests to expose the bug. Logic is valid, but couldn't replicate, until I told it understand what papaya is doing, then told it to switch to DashMap.

The failing test clearly shows:
BUG: Canonicalization failed with 'Canonicalized path was dropped' after
clear_cache().
This happens because the OnceLock still has a cached weak reference, but
clear_cache()
removed the strong reference from cache.paths.

Why DashMap exposed the bug:

• Papaya uses epoch-based memory reclamation - strong references aren't
  immediately dropped
• DashMap immediately drops entries when clear() is called - weak references
  immediately become invalid

[Boshen]

Ok I can't figure out a better to solve this, merging.