[v7.x] test: add regression test for invalid compressed fetch body

[mcollina]

This relates to...

Refs #5999

Rationale

v7.x already includes the fix for #5999 (registering the terminated listener only once per fetch and no longer removing it in onComplete), but it is missing a regression test.

This adds the same regression test that lands on main (and the v6 backport): a response that advertises content-encoding: gzip but delivers an invalid gzip body in a later packet than the headers must reject text() instead of hanging forever.

Notes on the test

The body must arrive in a separate packet than the headers for the regression to manifest, so the fetch controller's terminated listener is removed in onComplete before decompression fails.

Rather than a wall-clock setTimeout (flaky, adds a fixed delay), the body is deferred to a later event-loop tick with setImmediate. Two ticks deterministically split the headers and body into separate packets (a single tick coalesces them and does not reproduce the bug).

Verification

• test/fetch/encoding.js: 7/7 pass (new test ~3ms).
• standard lint: clean.

Status

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

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.11%. Comparing base (0c74bec) to head (a3db024).

Additional details and impacted files

@@           Coverage Diff           @@
##             v7.x    #6007   +/-   ##
=======================================
  Coverage   93.11%   93.11%           
=======================================
  Files         112      112           
  Lines       37366    37366           
=======================================
  Hits        34793    34793           
  Misses       2573     2573           

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