chore: add unit tests for pure functions + CI test job

- Unit tests for normalize_email_address, payload_matches_participant,
  format_search_result (9 test cases across 3 test classes)
- New 'test' job in CI workflow (runs before build)
- pytest.ini for pythonpath config

tests/test_server.py

@@ -0,0 +1,106 @@
+"""Unit tests for mcp-maildir server pure functions."""
+
+from server import (
+    normalize_email_address,
+    payload_matches_participant,
+    format_search_result,
+)
+
+
+class TestNormalizeEmailAddress:
+    def test_empty_value(self):
+        assert normalize_email_address("") == ""
+        assert normalize_email_address(None) == ""
+
+    def test_simple_email(self):
+        assert normalize_email_address("user@example.com") == "user@example.com"
+
+    def test_display_name_with_email(self):
+        assert normalize_email_address("John Doe <john@example.com>") == "john@example.com"
+
+    def test_whitespace_and_case(self):
+        assert normalize_email_address("  USER@Example.COM  ") == "user@example.com"
+
+    def test_invalid_email_fallback(self):
+        result = normalize_email_address("not-an-email")
+        # Returns stripped lowercase version of the input
+        assert result == "not-an-email"
+
+
+class TestPayloadMatchesParticipant:
+    def test_sender_match_normalized(self):
+        payload = {"sender": "alice@example.com"}
+        assert payload_matches_participant(payload, "alice@example.com") is True
+
+    def test_receiver_match_normalized(self):
+        payload = {"receiver": "bob@example.com"}
+        assert payload_matches_participant(payload, "bob@example.com") is True
+
+    def test_sender_raw_match(self):
+        payload = {"sender_raw": "alice@other.com"}
+        assert payload_matches_participant(payload, "alice@other.com") is True
+
+    def test_no_match(self):
+        payload = {"sender": "alice@example.com", "receiver": "bob@example.com"}
+        assert payload_matches_participant(payload, "carol@example.com") is False
+
+    def test_display_name_in_sender(self):
+        payload = {"sender": "Alice <alice@example.com>"}
+        assert payload_matches_participant(payload, "alice@example.com") is True
+
+    def test_display_name_in_receiver(self):
+        payload = {"receiver": "Bob Smith <bob@example.com>"}
+        assert payload_matches_participant(payload, "bob@example.com") is True
+
+    def test_empty_payload(self):
+        assert payload_matches_participant({}, "someone@example.com") is False
+
+    def test_case_insensitive(self):
+        payload = {"sender": "ALICE@EXAMPLE.COM"}
+        assert payload_matches_participant(payload, "alice@example.com") is True
+
+
+class TestFormatSearchResult:
+    def test_basic_formatting(self):
+        class MockPoint:
+            payload = {
+                "message_id": "<abc@def>",
+                "date": "2026-01-15T10:00:00",
+                "sender": "alice@example.com",
+                "receiver": "bob@example.com",
+                "subject": "Hello",
+                "attachments": ["file.pdf"],
+            }
+            score = 0.95
+
+        result = format_search_result(MockPoint())
+        assert result["message_id"] == "<abc@def>"
+        assert result["date"] == "2026-01-15T10:00:00"
+        assert result["sender"] == "alice@example.com"
+        assert result["receiver"] == "bob@example.com"
+        assert result["subject"] == "Hello"
+        assert result["attachments"] == ["file.pdf"]
+        assert result["score"] == 0.95
+
+    def test_empty_payload(self):
+        class MockPoint:
+            payload = None
+            score = None
+
+        result = format_search_result(MockPoint())
+        assert result["message_id"] is None
+        assert result["score"] is None
+        assert result["attachments"] == []
+
+    def test_missing_fields(self):
+        class MockPoint:
+            payload = {"message_id": "<123>"}
+            score = 0.5
+
+        result = format_search_result(MockPoint())
+        assert result["message_id"] == "<123>"
+        assert result["date"] is None
+        assert result["sender"] is None
+        assert result["receiver"] is None
+        assert result["subject"] is None
+        assert result["attachments"] == []