feat(M13): complete test fixtures, shared conftest, and coverage >= 90%

Finalize M13 test and coverage milestone:

tests/fixtures/pronote-6e.ics (new):
- Anonymized iCal fixture for Classe de 6e (3 VEVENTs: SVT lesson with
  homework block, Histoire-Géo modified lesson, all-day school outing)
- Same structure as pronote-4e.ics, no secrets or real data

tests/conftest.py:
- Added sample_message fixture (Message with MessageType.INFORMATION)
- Integrated sample_message into pronote_data fixture (messages=[sample_message])
- Sphinx/reST docstring with :return: and :rtype:

.gitignore:
- Fixed typo: .worktress/ -> .worktrees/ (line 56)

pyproject.toml:
- Added ".worktrees" to ruff extend-exclude to prevent ruff format --check .
  from scanning worktree files

TODO.md:
- Checked M13 items: tests/fixtures/ and tests/conftest.py

Validation: 627 tests pass, coverage 95.67% (threshold 90%), ruff/mypy/
bandit/pre-commit all green.

Co-authored-by: opencode/coder <coder@agents.invalid>

tests/conftest.py

@@ -14,6 +14,7 @@ from pydantic import SecretStr
 from pronote_sync.config.settings import CalDAVSettings
 from pronote_sync.models.agenda import Lesson, LessonStatus, SchoolEvent, SchoolEventKind
 from pronote_sync.models.homework import Homework
+from pronote_sync.models.message import Message, MessageType
 from pronote_sync.models.pronote import PronoteData
 
 
@@ -85,19 +86,38 @@ def caldav_settings() -> CalDAVSettings:
     )
 
 
+@pytest.fixture
+def sample_message() -> Message:
+    """Message Pronote pour les tests.
+
+    :return: Message Pronote de test.
+    :rtype: Message
+    """
+    return Message(
+        id="msg-001",
+        type=MessageType.INFORMATION,
+        title="Information de rentrée",
+        content="La rentrée est prévue le 1er septembre.",
+        author="Administration",
+        date=datetime(2026, 1, 15, 9, 0),
+        read=False,
+    )
+
+
 @pytest.fixture
 def pronote_data(
     sample_lesson: Lesson,
     sample_cancelled_lesson: Lesson,
     sample_homework: Homework,
     sample_school_event: SchoolEvent,
+    sample_message: Message,
 ) -> PronoteData:
-    """Données Pronote de test avec des cours, devoirs et événements."""
+    """Données Pronote de test avec des cours, devoirs, événements et messages."""
     return PronoteData(
         lessons=[sample_lesson, sample_cancelled_lesson],
         homeworks=[sample_homework],
         school_events=[sample_school_event],
-        messages=[],
+        messages=[sample_message],
         target_date=date(2026, 1, 15),
         generated_at=datetime(2026, 1, 15, 0, 0),
     )