fix: avoid prototype collisions in parseHeaders

[mcollina]

Fix parseHeaders() for response headers and trailers that shadow Object.prototype.

Adds regression tests.

[ronag]

Perf impact?

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.94%. Comparing base (feef62b) to head (108e939).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4923      +/-   ##
==========================================
+ Coverage   92.93%   92.94%   +0.01%     
==========================================
  Files         112      112              
  Lines       35711    35709       -2     
==========================================
+ Hits        33188    33191       +3     
+ Misses       2523     2518       -5     

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[tsctx]

Perf impact?

• main

> node ./benchmarks/core/parse-headers.mjs
clk: ~3.21 GHz
cpu: AMD Ryzen 7 8845HS w/ Radeon 780M Graphics
runtime: node 25.8.1 (x64-win32)

benchmark                   avg (min … max) p75 / p99    (min … top 1%)
------------------------------------------- -------------------------------
• parseHeaders
------------------------------------------- -------------------------------
parseHeaders                   6.65 µs/iter   6.72 µs     █  ██
                        (6.20 µs … 7.81 µs)   7.34 µs   ███  ██
                    (609.53  b …   2.59 kb)   2.37 kb █▁████▁█████▁▁▁█▁▁▁▁█

parseHeaders (irregular)       6.80 µs/iter   7.02 µs ██
                        (6.30 µs … 9.27 µs)   7.92 µs ██▂▂
                    (  2.59 kb …   2.60 kb)   2.59 kb ████▆▁▁▁▁▆▁▁▁▆▆▁▆▁▁▁▆

• this PR

> node ./benchmarks/core/parse-headers.mjs
clk: ~3.23 GHz
cpu: AMD Ryzen 7 8845HS w/ Radeon 780M Graphics
runtime: node 25.8.1 (x64-win32)

benchmark                   avg (min … max) p75 / p99    (min … top 1%)
------------------------------------------- -------------------------------
• parseHeaders
------------------------------------------- -------------------------------
parseHeaders                  16.86 µs/iter  15.00 µs █
                     (14.20 µs … 303.20 µs)  36.40 µs █
                    (648.00  b … 260.46 kb)   5.33 kb █▆▂▁▁▁▂▁▁▁▁▁▂▂▁▁▁▁▁▁▁

parseHeaders (irregular)      17.05 µs/iter  16.79 µs   █     █
                      (15.00 µs … 27.50 µs)  18.01 µs ▅▅█ ▅   █▅  ▅ ▅     ▅
                    (  1.06 kb …   5.06 kb)   4.26 kb ███▁█▁▁▁██▁▁█▁█▁▁▁▁▁█

[mcollina]

Fixing it.

[mcollina]

ptal @tsctx

[mcollina]

Should have no regressions now.

[tsctx]

lgtm