Widen the with_extensions annotation to both hooks

`with_extensions` accepts a bundle implementing either hook — the runtime check
tests against both protocols, `test_with_extensions_accepts_a_planner_only_extension`
pins it, and the FFI guide's `PlannerOf` adapter recommends contributing only the
planner half. The annotation named `SessionExtensionExportable` alone, so a type
checker rejected the very shape the guide tells authors to write.

Two doc comments also went stale. `test_with_extensions_no_extensions_keeps_an_installed_planner`
still described phase two running and rebinding an existing planner, which the
no-op guard now skips outright, and `__datafusion_codec_id__` listed a
`_install_extensions` method that never existed under that name.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: timsaucer

crates/core/src/context.rs

@@ -1435,11 +1435,11 @@ impl PySessionContext {
     /// decode. See [`SESSION_CODEC_ID_PREFIX`].
     ///
     /// Handles derived from one session — `with_python_udf_inlining`,
-    /// `with_logical_extension_codec`, `_install_extensions` — report the same
-    /// id even though their codec chains differ, so installing two of them on
-    /// one target is refused. That is the intended answer: they share a
-    /// `state_ref`, so their payloads would resolve against the same session
-    /// and are indistinguishable on decode. Every derivation shares the
+    /// `with_logical_extension_codec`, [`Self::_install_extension_codecs`] —
+    /// report the same id even though their codec chains differ, so installing
+    /// two of them on one target is refused. That is the intended answer: they
+    /// share a `state_ref`, so their payloads would resolve against the same
+    /// session and are indistinguishable on decode. Every derivation shares the
     /// session for exactly this reason; `enable_url_table` is the one that does
     /// not, and it is tracked as a bug.
     #[getter]

python/datafusion/context.py

@@ -1812,7 +1812,7 @@ def set_query_planner(self, planner: QueryPlannerExportable | _PyCapsule) -> Non
         self.ctx.set_query_planner(planner)
 
     def with_extensions(
-        self, *extensions: SessionExtensionExportable
+        self, *extensions: SessionExtensionExportable | SessionPlannerExportable
     ) -> SessionContext:
         """Create a new session context with the given extension bundles.
 

python/tests/test_context.py

@@ -971,9 +971,9 @@ def test_with_extensions_accepts_no_extensions(ctx):
 def test_with_extensions_no_extensions_keeps_an_installed_planner(ctx):
     """The empty case must not disturb a planner the session already has.
 
-    Phase two still runs with nothing to install, which rebinds an existing
-    FFI planner to the codec chains — unchanged here, so the planner has to
-    come through intact.
+    A call that installs no codec and no planner skips the planner commit
+    entirely, so the installed planner keeps the chains it was bound to and the
+    session still plans through it.
     """
     extension = _CodecOnlyExtension()
     installed = ctx.with_extensions(extension, _PlannerExtension())