docs: tighten codec dispatch test docstrings

The previous docstrings claimed the tests verify "PythonLogicalCodec
delegates non-Python UDFs to the inner codec." That's
forward-looking — the codecs currently delegate every UDF
unconditionally, so the test would behave identically for Python and
non-Python UDFs.

Rewrite to describe what the test actually proves: the dispatch chain
`PyLogicalPlan.to_bytes -> session.logical_codec -> PythonLogicalCodec
-> FFI -> user impl` (and the physical mirror) forwards correctly,
observable via the user codec's atomic counter incrementing after one
encode pass.

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

Author: timsaucer

examples/datafusion-ffi-example/python/tests/_test_logical_extension_codec.py

@@ -41,9 +41,20 @@ def test_ffi_logical_codec_install_and_export():
 
 
 def test_ffi_logical_codec_consulted_on_udf_encode():
-    """Serializing a plan that references an FFI-imported scalar UDF
-    routes through the installed codec's `try_encode_udf` path —
-    PythonLogicalCodec delegates non-Python UDFs to the inner codec."""
+    """Serializing through ctx.logical_codec() routes try_encode_udf to
+    the user-installed FFI codec.
+
+    Verifies the dispatch chain
+    `PyLogicalPlan.to_bytes -> session.logical_codec ->
+    PythonLogicalCodec -> FFI_LogicalExtensionCodec -> user impl`
+    is wired correctly. The user codec's atomic counter increments
+    after a serialization pass, proving every hop forwards.
+
+    Does not test any Python-UDF-specific dispatch — PythonLogicalCodec
+    currently delegates all UDF encoding to its inner codec
+    unconditionally. Python-vs-other branching lands when in-band
+    scalar UDF encoding is added.
+    """
     ctx, codec = _setup_session_with_codec()
     df = ctx.sql("SELECT abs(-1) AS x")
     plan = df.logical_plan()

examples/datafusion-ffi-example/python/tests/_test_physical_extension_codec.py

@@ -41,10 +41,16 @@ def test_ffi_physical_codec_install_and_export():
 
 
 def test_ffi_physical_codec_consulted_on_udf_encode():
-    """Serializing an ExecutionPlan that references an FFI-imported
-    scalar UDF routes `try_encode_udf` through the installed physical
-    codec — PythonPhysicalCodec delegates non-Python UDFs to the inner
-    codec."""
+    """Serializing through ctx.physical_codec() routes try_encode_udf to
+    the user-installed FFI codec.
+
+    Mirror of the logical-side dispatch test: verifies
+    `PyExecutionPlan.to_bytes -> session.physical_codec ->
+    PythonPhysicalCodec -> FFI_PhysicalExtensionCodec -> user impl`
+    forwards correctly. Does not test Python-UDF-specific dispatch —
+    PythonPhysicalCodec currently delegates all UDF encoding to its
+    inner codec unconditionally.
+    """
     ctx, codec = _setup_session_with_codec()
     df = ctx.sql("SELECT abs(a) AS x FROM t")
     plan = df.execution_plan()