test: pin keyword-only construction; scope the rollback promise

Add a test that SessionExtensionComponents rejects positional
arguments, so dropping kw_only=True fails the suite. Every existing
caller passes keywords and would stay green without it.

Drop the absence assertions from the extension protocol import test.
Re-exporting a name is additive and breaks no caller, so asserting a
name is missing only adds friction for a later deliberate export.
Keep the positive check that each protocol imports from
datafusion.extensions.

The contributor guide promised that a raising bundle leaves the
session as it was without the carve-out for writes a hook makes to
the context it is handed. Add it with a ref to the bundles guide,
which is where the exception is explained.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: timsaucer

docs/source/contributor-guide/ffi-internals.md

@@ -116,7 +116,8 @@ prefer `with_extensions` — is documented at {ref}`planner_codec_rebinding`.
 ## Why `with_extensions` commits last
 
 `with_extensions` promises that a bundle which raises leaves the session as it
-was. Keeping that promise is an ordering constraint on the implementation, not
+was, apart from anything a hook writes to the context it is handed
+({ref}`extension_bundles_transaction`). Keeping that promise is an ordering constraint on the implementation, not
 a property of any one step, because the planner is bound on the shared
 `SessionState` rather than on the returned handle.
 

python/tests/test_context.py

@@ -1086,6 +1086,21 @@ def test_session_extension_components_rejects_a_single_codec(field):
         SessionExtensionComponents(**{field: codec})
 
 
+def test_session_extension_components_is_keyword_only():
+    codecs = (
+        _NamedCodec(
+            SessionContext().__datafusion_logical_extension_codec__(),
+            "my_library.logical",
+        ),
+    )
+
+    with pytest.raises(TypeError, match=r"positional argument"):
+        SessionExtensionComponents(codecs)
+
+    components = SessionExtensionComponents(logical_extension_codecs=codecs)
+    assert components.logical_extension_codecs == codecs
+
+
 def test_session_extension_components_rejects_a_string():
     """A str is iterable, so it needs refusing on its own.
 

python/tests/test_imports.py

@@ -201,13 +201,10 @@ def test_import_from_functions_submodule():
         "SessionPlannerExportable",
     ],
 )
-def test_extension_protocols_live_only_in_extensions(name):
-    import datafusion.context
+def test_extension_protocols_import_from_extensions(name):
     import datafusion.extensions
 
     assert getattr(datafusion.extensions, name).__module__ == "datafusion.extensions"
-    assert not hasattr(datafusion.context, name)
-    assert not hasattr(datafusion, name)
 
 
 def test_classes_are_inheritable():