fix: keep callback faults out of the frame-decode fallback

Review follow-up on the previous two commits.

Widening the control-message except tuple put the callback dispatch
inside it, so an _on_new_cycle() that raised ValueError, TypeError or
AttributeError sent a perfectly good control packet to the legacy PNG
decoder -- which reported it as an image decode error and buried the
real fault. Split the two: whether the payload parses as JSON decides
frame vs control message, a second guard covers reading the fields of an
attacker-shaped body, and the callback fires outside both. It still
cannot kill the receive thread; the loop's own handler catches it, and
now says what actually went wrong.

The logo download's temp file was a fixed "<name>.part". Two plugins
asking for the same logo at once would interleave writes into it,
publish the mixture, or delete each other's partial. mkstemp gives each
download its own name in the same directory, so os.replace stays atomic.
Its descriptor is adopted by fdopen before the request runs, since a
request that raises before the write would otherwise leak the fd --
quietly, because load_logo_with_download swallows that.

Two test fixes: the oversized-frame test replaced PIL.Image.open
process-wide, the same hazard the clock helper documents, and Ruff B007
on an unused loop variable.

Full suite: 3355 passed, coverage 54%.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NohXi78cwsAKtN1sCfxjUh

test/test_logo_helper.py

@@ -16,6 +16,7 @@ Regression coverage for two fixed bugs:
 """
 
 import logging
+import tempfile
 from pathlib import Path
 from unittest.mock import MagicMock, patch
 
@@ -290,6 +291,38 @@ class TestDownloadLogo:
         assert not path.exists()
         assert list(tmp_path.glob("*.part")) == []
 
+    def test_concurrent_downloads_do_not_share_a_temp_file(self, helper, tmp_path):
+        # Two plugins can ask for the same logo at once. A fixed
+        # "<name>.part" would let them interleave writes into one file and
+        # publish the mixture; each download gets its own temp name.
+        path = tmp_path / "PHI.png"
+        seen = []
+        real_mkstemp = tempfile.mkstemp
+
+        def record(*args, **kwargs):
+            fd, name = real_mkstemp(*args, **kwargs)
+            seen.append(name)
+            return fd, name
+
+        with patch("src.common.logo_helper.tempfile.mkstemp", side_effect=record):
+            helper.session.get = MagicMock(return_value=fake_response(png_bytes()))
+            helper._download_logo("http://x/logo.png", path)
+            helper.session.get = MagicMock(return_value=fake_response(png_bytes()))
+            helper._download_logo("http://x/logo.png", path)
+
+        assert len(seen) == 2 and seen[0] != seen[1]
+        assert path.exists()
+        assert list(tmp_path.glob("*.part")) == []  # both cleaned up
+
+    def test_request_failure_leaves_no_temp_file(self, helper, tmp_path):
+        # mkstemp creates the file up front, so an error before any bytes
+        # arrive still has something to clean up.
+        helper.session.get = MagicMock(
+            side_effect=requests.RequestException("connection reset"))
+        with pytest.raises(requests.RequestException):
+            helper._download_logo("http://x/logo.png", tmp_path / "PHI.png")
+        assert list(tmp_path.glob("*")) == []
+
     def test_non_image_response_is_deleted_and_raises(self, helper, tmp_path):
         # Regression: undecodable bytes stayed on disk, so every later
         # load_logo() call hit the corrupt file instead of re-downloading.