fix(imap): handle multi-address headers in _parse_singular_addr (#15006)

Replace the RuntimeError with a warning + first-address fallback so a
single email whose From header contains multiple addresses no longer
crashes the entire IMAP sync task. Also add regression tests covering:

- #14963: RFC 5322 quoted display names with commas (e.g. "Schlüter,
Sabine" <s@x>) parsed as one address, not two.
- #14964: multi-address headers warn instead of raising.

Closes #14964
Refs #14963

test/unit_test/data_source/test_imap_connector_addr_parsing.py

@@ -0,0 +1,101 @@
+#
+#  Copyright 2025 The InfiniFlow Authors. All Rights Reserved.
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+#
+
+"""Regression tests for IMAP connector address parsing.
+
+Covers:
+- #14963: ``_parse_addrs`` must respect RFC 5322 quoted display names that
+  contain commas (e.g. ``"Schlüter, Sabine" <s@x>``) and return one address,
+  not two.
+- #14964: ``_parse_singular_addr`` must not raise on multi-address headers;
+  it should warn and return the first address so a single odd email does not
+  crash the entire sync.
+"""
+
+import logging
+
+import pytest
+
+from common.data_source.imap_connector import _parse_addrs, _parse_singular_addr
+
+pytestmark = pytest.mark.p2
+
+
+class TestParseAddrs:
+    def test_empty_header_returns_empty_list(self):
+        assert _parse_addrs("") == []
+
+    def test_simple_address(self):
+        assert _parse_addrs("user@example.com") == [("", "user@example.com")]
+
+    def test_address_with_display_name(self):
+        assert _parse_addrs("Alice <alice@example.com>") == [
+            ("Alice", "alice@example.com")
+        ]
+
+    def test_quoted_display_name_with_comma_returns_single_address(self):
+        # #14963: the bug was that ``split(",")`` produced two bogus tuples.
+        # ``getaddresses`` must keep the quoted "Last, First" as one unit.
+        result = _parse_addrs('"Schlüter, Sabine" <sabine.schlueter@ihklw.de>')
+        assert result == [("Schlüter, Sabine", "sabine.schlueter@ihklw.de")]
+
+    def test_multiple_addresses_comma_separated(self):
+        result = _parse_addrs("a@example.com, b@example.com")
+        assert result == [("", "a@example.com"), ("", "b@example.com")]
+
+    def test_multiple_addresses_with_quoted_comma_in_name(self):
+        result = _parse_addrs(
+            '"Wilkens, Michael" <m@example.com>, "Müller, Hans" <h@example.com>'
+        )
+        assert result == [
+            ("Wilkens, Michael", "m@example.com"),
+            ("Müller, Hans", "h@example.com"),
+        ]
+
+
+class TestParseSingularAddr:
+    def test_empty_header_returns_unknown(self):
+        assert _parse_singular_addr("") == ("Unknown", "unknown@example.com")
+
+    def test_single_address(self):
+        assert _parse_singular_addr("Alice <alice@example.com>") == (
+            "Alice",
+            "alice@example.com",
+        )
+
+    def test_quoted_comma_display_name_does_not_raise(self):
+        # #14963 cascade: before the fix, ``_parse_addrs`` returned two bogus
+        # tuples and ``_parse_singular_addr`` then raised RuntimeError.
+        assert _parse_singular_addr(
+            '"Schlüter, Sabine" <sabine.schlueter@ihklw.de>'
+        ) == ("Schlüter, Sabine", "sabine.schlueter@ihklw.de")
+
+    def test_multi_address_header_warns_and_returns_first(self, caplog):
+        # #14964: a legitimately multi-address From header must not crash sync.
+        header = '"User A" <a@example.com>, "User B" <b@example.com>'
+        with caplog.at_level(logging.WARNING):
+            result = _parse_singular_addr(header)
+        assert result == ("User A", "a@example.com")
+        assert any(
+            "Multiple addresses" in rec.message for rec in caplog.records
+        ), f"expected warning about multiple addresses, got: {caplog.records}"
+
+    def test_multi_address_header_does_not_raise(self):
+        # Explicit guard: no RuntimeError should propagate.
+        try:
+            _parse_singular_addr("a@example.com, b@example.com")
+        except RuntimeError as e:  # pragma: no cover - guard only
+            pytest.fail(f"_parse_singular_addr unexpectedly raised: {e}")