#866 hardened the comment-persistence read-back into a full-path _belongs()
check that pins scheme+host+port origin plus the full path, to block
look-alike-host and same-host decoy-prefix attacks. That check compared the
path case-sensitively: EXPECTED_REPO_SLUG is taken verbatim from
GITEA_API_BASE and can be mixed-case (e.g. "USC/uconnect"), while Gitea
canonicalizes the returned pull_request_url's owner/repo segment to
lowercase. A successful 201 write on a mixed-case-slug repo therefore
false-negatived ("did not land on a pull request") even though the comment
genuinely landed — fail-closed on a real success, not a false success, but
broken for legitimate mixed-case-slug repos.
Lowercase both sides of the path comparison inside _belongs (path and
expected_path) while leaving the origin tuple comparison, _origin_and_path's
scheme+host lowercasing, and the full (non-suffix) path match untouched — the
look-alike-host and decoy-prefix protections from #866 are unchanged; only
the repo-slug segment's case sensitivity is relaxed, which is safe because
Gitea repo slugs are case-insensitive and the remainder of the path is
numeric.
Adds a red-first regression case (comment-mixed-case-slug) modeling a
mixed-case EXPECTED_REPO_SLUG against a Gitea-canonicalized lowercase
pull_request_url, which fails against pre-fix code and passes after.
Closes#875
