fix: avoid deduplication key collisions

[mcollina]

Description

Fix deduplication key collisions in the deduplicate interceptor.

The old key builder concatenated headers with : and = delimiters without escaping them, so distinct header sets could produce the same in-flight deduplication key.

This change switches makeDeduplicationKey() to a structured JSON serialization of:

• origin
• method
• path
• sorted header entries

That preserves deterministic ordering while avoiding delimiter-based collisions.

Changes

• replace delimiter-based deduplication key construction with structured serialization
• add a unit test covering the reported header collision
• add an interceptor test proving those requests no longer deduplicate incorrectly

Testing

• node --test test/cache-interceptor/cache-utils.js
• ./node_modules/.bin/borp -p "test/interceptors/deduplicate.js"
• ./node_modules/.bin/eslint lib/util/cache.js test/cache-interceptor/cache-utils.js test/interceptors/deduplicate.js

Closes: #5012

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.93%. Comparing base (4f4bf4f) to head (2f76703).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5017      +/-   ##
==========================================
+ Coverage   92.86%   92.93%   +0.06%     
==========================================
  Files         110      110              
  Lines       35788    35743      -45     
==========================================
- Hits        33235    33217      -18     
+ Misses       2553     2526      -27     

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