fix(matrix): fail-closed approval reaction auth (parity with Telegram)

[teknium1]

The bug (confirmed live on main)

gateway/platforms/matrix.py:2239 gated approval reactions with:

if self._allowed_user_ids and sender not in self._allowed_user_ids:
    return  # ignore unauthorized

When MATRIX_ALLOWED_USERS is empty, self._allowed_user_ids is falsy, the
whole guard short-circuits, and any user in the room can resolve a
dangerous-command approval prompt — fail-open.

The fix

_allow_all = os.getenv("GATEWAY_ALLOW_ALL_USERS", "").lower() in {"true", "1", "yes"}
if not _allow_all and not (self._allowed_user_ids and sender in self._allowed_user_ids):
    return  # ignore unauthorized

Fail-closed by default; the deliberate-open-access operator
(GATEWAY_ALLOW_ALL_USERS=true) is explicitly preserved. Mirrors the
already-shipped Telegram callback-auth shape (_is_callback_user_authorized,
telegram.py:558 — same GATEWAY_ALLOW_ALL_USERS fallback).

No regression — every supported config preserved

Configuration	Before	After
MATRIX_ALLOWED_USERS set (solo/team operator)	works	works (unchanged)
Paired via /pair	works	works (unchanged)
GATEWAY_ALLOW_ALL_USERS=true (deliberate open)	any user OK	any user OK (preserved)
No allowlist, no allow-all	any room member can approve ⚠ BUG	fail-closed

The only behavior that changes is the last row — and those senders can't pass
Layer-1 admission (_is_user_authorized defaults to deny) to issue a command
in the first place, so no legitimate user loses anything. The bug was an
inconsistency where an unauthorized user could still approve a prompt they
were never authorized to touch.

Verification

tests/gateway/test_matrix_approval_reaction_fail_closed.py — 4 tests, all
pass on current main:

• test_no_allowlist_no_allow_all_denies — the bug, now closed
• test_no_allowlist_allow_all_permits — no-regression guard (allow-all operator keeps open approvals)
• test_listed_sender_permits / test_unlisted_sender_denies — allowlist path unchanged

Part of the approval-gate-bypass cluster (issue #32877 names the single-gate
problem; this closes the Matrix reaction-auth surface). Salvage of #30062.

Closes #30062

Co-authored-by: EloquentBrush0x 283442588+EloquentBrush0x@users.noreply.github.com

[github-actions]

🔎 Lint report: fix/32877-matrix-approval-fail-closed vs origin/main

ruff

Total: 0 on HEAD, 0 on base (➖ 0)

🆕 New issues: none

✅ Fixed issues: none

Unchanged: 0 pre-existing issues carried over.

ty (type checker)

Total: 9437 on HEAD, 9435 on base (🆕 +2)

🆕 New issues (2):

Rule	Count
unresolved-import	1
unresolved-attribute	1

First entries

tests/gateway/test_matrix_approval_reaction_fail_closed.py:16: [unresolved-import] unresolved-import: Cannot resolve imported module `pytest`
tests/gateway/test_matrix_approval_reaction_fail_closed.py:88: [unresolved-attribute] unresolved-attribute: Unresolved attribute `resolve_gateway_approval` on type `ModuleType`

✅ Fixed issues: none

Unchanged: 4899 pre-existing issues carried over.

Diagnostics are surfaced as warnings — this check never fails the build.

[alt-glitch]

Competing with open PRs #30062 and #33328 (both fix the same Matrix fail-open approval auth bug). This PR implements the fix differently — using GATEWAY_ALLOW_ALL_USERS env var for parity with Telegram's callback-auth shape.