Header equality raises TypeError for non-iterable operands #214

Comparing a public event's headers with a non-iterable raises TypeError instead of following Python's equality fallback. This affects ordinary sentinel comparisons and prevents unittest.mock.ANY from handling a reflected comparison.

Reproduced on master at 62c5068c971579d61fa1b55373390e12f25fd856, macOS arm64, Python 3.12.7:

from unittest.mock import ANY
import h11

headers = h11.Request(
    method="GET", target="/", headers=[("Host", "example.com")]
).headers

print(headers == None)  # TypeError: 'NoneType' object is not iterable
print(headers == ANY)   # TypeError: '_ANY' object is not iterable

Expected results are False and True, respectively. Headers.__eq__ currently calls list(other) for every operand. An unsupported operand should allow Python to try the other operand's comparison method before falling back to identity.

A focused repair checks for Python's iterator or legacy sequence protocol and returns NotImplemented when neither is present. Existing list, tuple, iterator, Headers, and __getitem__-only comparisons retain their behavior. Exceptions raised by implemented iteration protocols must still propagate, including an internal TypeError from __iter__.

The original implementation fails four new regression cases, while six tests pass. The prepared repair passes the complete 86-test suite on Python 3.10.19, 3.11.14, 3.12.7, 3.13.9 and 3.14.0. Black, isort and strict mypy pass. Original and repaired statement/branch coverage both report 98.2% with the same 19 uncovered statements; the new branches are covered. Python 3.8, 3.9 and PyPy were not tested locally.

AI disclosure: Claude Code assisted with investigation, implementation, testing and this report. A separate automated reviewer identified an exception-propagation gap in an initial draft; a failing regression was added and the final revision was reviewed without a remaining blocker. No maintainer acceptance is claimed.