docs(context): trim codec docstrings, reference Exportable protocols

Drop prose restatement of the type union for `with_logical_extension_codec`
and `with_physical_extension_codec`. Keep the dunder name (not visible from
the type hint) and cross-link the `LogicalExtensionCodecExportable` /
`PhysicalExtensionCodecExportable` protocols so Sphinx resolves them.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: timsaucer

python/datafusion/context.py

@@ -1938,10 +1938,9 @@ def with_logical_extension_codec(
     ) -> SessionContext:
         """Create a new session context with specified codec.
 
-        This only supports codecs that have been implemented using the
-        FFI interface. ``codec`` must either be a raw ``FFI_LogicalExtensionCodec``
-        ``PyCapsule`` or an object exposing
-        ``__datafusion_logical_extension_codec__``.
+        Only FFI codecs are supported. Pass any object implementing
+        ``__datafusion_logical_extension_codec__`` (see
+        :py:class:`~datafusion.user_defined.LogicalExtensionCodecExportable`).
         """
         new_internal = self.ctx.with_logical_extension_codec(codec)
         new = SessionContext.__new__(SessionContext)
@@ -1957,10 +1956,9 @@ def with_physical_extension_codec(
     ) -> SessionContext:
         """Create a new session context with the specified physical codec.
 
-        This only supports codecs that have been implemented using the
-        FFI interface. ``codec`` must either be a raw
-        ``FFI_PhysicalExtensionCodec`` ``PyCapsule`` or an object exposing
-        ``__datafusion_physical_extension_codec__``.
+        Only FFI codecs are supported. Pass any object implementing
+        ``__datafusion_physical_extension_codec__`` (see
+        :py:class:`~datafusion.user_defined.PhysicalExtensionCodecExportable`).
         """
         new_internal = self.ctx.with_physical_extension_codec(codec)
         new = SessionContext.__new__(SessionContext)