fix(cache): refetch when 304 adds vary fields

[mcollina]

This relates to...

N/A

Rationale

Foreground 304 Not Modified revalidation bypasses CacheHandler's metadata update path. If that response introduces a valid Vary field that is absent from the stored entry, the cached variant metadata remains incomplete.

Changes

Features

N/A

Bug Fixes

• Detect newly introduced Vary request-header fields during synchronous revalidation.
• Evict the stale entry and refetch through CacheHandler so the replacement is stored with complete variant metadata.
• Add regression coverage for synchronous and asynchronous cache stores.

Breaking Changes and Deprecations

N/A

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• Benchmarked (optional)
• Documented (no documentation changes needed)
• Review ready
• In review
• Merge ready

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.28571% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.46%. Comparing base (fc3450d) to head (ee8bebb).

Files with missing lines	Patch %	Lines
lib/interceptor/cache.js	94.28%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5732      +/-   ##
==========================================
- Coverage   93.47%   93.46%   -0.01%     
==========================================
  Files         110      110              
  Lines       38908    38942      +34     
==========================================
+ Hits        36368    36397      +29     
- Misses       2540     2545       +5     

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