fix(decompress): preserve response trailers

[Ram-blip]

This relates to...

No existing issue. This fixes response trailers being discarded by interceptors.decompress() after successful decompression.

Rationale

interceptors.decompress() delays response completion until the decompression stream finishes. Its completion callbacks previously forwarded an empty trailers object, causing valid HTTP response trailers to be lost.

The trailers received by onResponseEnd() must be retained across the asynchronous decompression boundary and forwarded when decompression completes.

Changes

• Store normalized response trailers while decompression is finishing.
• Forward trailers through both the single-decoder and chained-decoder completion paths.
• Add regression coverage for gzip responses with trailers.
• Extend the multiple-content-encoding test to verify trailer preservation.

Features

N/A

Bug Fixes

• Preserve HTTP response trailers when using interceptors.decompress().
• Preserve trailers for responses with multiple content encodings.

Breaking Changes and Deprecations

N/A

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• Benchmarked (optional)
• Documented
• Review ready
• In review
• Merge ready

Tests

node --test test/interceptors/decompress.js
npm run test:interceptors
npm run lint

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.47%. Comparing base (1383989) to head (a84de8f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5572   +/-   ##
=======================================
  Coverage   93.47%   93.47%           
=======================================
  Files         110      110           
  Lines       37560    37563    +3     
=======================================
+ Hits        35108    35113    +5     
+ Misses       2452     2450    -2     

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[mcollina]

lgtm