fix: validate CHAT_CLIENT_IP_HEADER as HTTP field-name at boot (fixes #745)

[RobGenins]

CHAT_CLIENT_IP_HEADER was lowercased and stripped but not validated as an HTTP field-name. A non-ASCII value passed config parsing and booted, then crashed every rate-limited route with UnicodeEncodeError at Latin-1 header lookup.

Changes (v2, replacing closed #837 and superseding #857):

• Validate that non-empty CHAT_CLIENT_IP_HEADER matches the HTTP token grammar (RFC 9110 section 5.1) via .isascii() and a token regex.
• Empty string remains the documented opt-out
• Regression test for non-ASCII, colon, space, comma, and valid header values

Also fixes #789 (sz.py docstring omits src/limit.py from the core file set).

All 779 tests pass, 1 skipped.

Fixes: #745, #789

[github-actions]

Open pull requests citing the same issues:

• fix: doc fixes — sz.py lists limit.py, README drops stale CHAT_IDLE_SECONDS ref (fixes #789, #779) #801 (also cites sz.py docstring omits src/limit.py from the core file set #789)
• docs(sz): list src/limit.py in the core file set docstring #790 (also cites sz.py docstring omits src/limit.py from the core file set #789)

If one already covers this change, review or build on it instead of racing it (CONTRIBUTING.md "Overlapping work").

[Minh3132]

There is an unrelated, already-owned fix bundled into this head that is not named in the PR scope. Besides the CHAT_CLIENT_IP_HEADER validation and the explicitly declared #789 sz.py docstring change, the diff also rewrites README's CHAT_STILLBORN_SECONDS row from the nonexistent CHAT_IDLE_SECONDS reference to the 7-day IDLE_SECONDS window. That is the exact #779 documentation defect, and it already has its own open focused PR (#848).

This matters beyond tidiness: CONTRIBUTING says to keep each PR focused, avoid unrelated changes, and review/build on existing overlapping work rather than racing it. Because #864 neither cites #779/#848 nor explains why it needs that README hunk, queue-guard cannot flag the overlap; merging this head would silently consume another open PR's issue while the title/body say this PR fixes #745/#789.

Please drop the README CHAT_STILLBORN_SECONDS hunk from this branch (or, if there is a real dependency on #779, explicitly declare and justify that overlap). The header-name validation itself does not depend on the stillborn-window documentation.

[RobGenins]

Rebased and cleaned: dropped unrelated README hunk and bazel-v0.14 reversion from branch. Replaced by fresh PR.