feat: add PipelineWarning for non-blocking pipeline errors (M10-U0)

Add PipelineWarning(PronoteSyncError) to the canonical error hierarchy.
This non-blocking warning type is used by the XMPP channel (and future
channels) to signal recoverable failures without breaking the pipeline.

- PipelineWarning inherits from PronoteSyncError, not Warning builtin
- Constructor: (message, step=None) with recoverable=True
- 8 unit tests covering inheritance, raising, catching, attributes

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

tests/unit/test_errors.py

@@ -0,0 +1,88 @@
+"""Tests unitaires pour la hiérarchie des erreurs du pipeline.
+
+Ce module valide les classes d'erreur définies dans pronote_sync.errors.
+"""
+
+import pytest
+
+from pronote_sync.errors import PipelineWarning, PronoteSyncError
+
+
+def test_pipeline_warning_inherits_pronote_sync_error() -> None:
+    """Vérifie que PipelineWarning hérite de PronoteSyncError.
+
+    :return: None
+    :rtype: None
+    """
+    assert isinstance(PipelineWarning("msg"), PronoteSyncError)
+
+
+def test_pipeline_warning_not_warning_builtin() -> None:
+    """Vérifie que PipelineWarning n'hérite pas de la classe Warning intégrée.
+
+    :return: None
+    :rtype: None
+    """
+    assert not isinstance(PipelineWarning("msg"), Warning)
+
+
+def test_pipeline_warning_message_stored() -> None:
+    """Vérifie que le message est stocké et accessible via str(exc).
+
+    :return: None
+    :rtype: None
+    """
+    exc = PipelineWarning("msg")
+    assert str(exc) == "msg"
+    assert exc.args[0] == "msg"
+
+
+def test_pipeline_warning_step_default_none() -> None:
+    """Vérifie que step est None par défaut.
+
+    :return: None
+    :rtype: None
+    """
+    assert PipelineWarning("msg").step is None
+
+
+def test_pipeline_warning_step_set() -> None:
+    """Vérifie que step peut être défini via le constructeur.
+
+    :return: None
+    :rtype: None
+    """
+    assert PipelineWarning("msg", step="xmpp").step == "xmpp"
+
+
+def test_pipeline_warning_recoverable_true() -> None:
+    """Vérifie que recoverable est toujours True pour PipelineWarning.
+
+    :return: None
+    :rtype: None
+    """
+    assert PipelineWarning("msg").recoverable is True
+
+
+def test_pipeline_warning_is_raisable() -> None:
+    """Vérifie que PipelineWarning peut être levée.
+
+    :return: None
+    :rtype: None
+    """
+    with pytest.raises(PipelineWarning, match="msg"):
+        raise PipelineWarning("msg")
+
+
+def test_pipeline_warning_caught_by_pronote_sync_error() -> None:
+    """Vérifie qu'une PipelineWarning est attrapée par un except PronoteSyncError.
+
+    :return: None
+    :rtype: None
+    """
+    try:
+        raise PipelineWarning("msg")
+    except PronoteSyncError:
+        assert True
+    else:
+        raise AssertionError("PipelineWarning should have been caught by PronoteSyncError")