test(pickle): tighten refusal assertion from Exception to RuntimeError

`pytest.raises(Exception, ...)` matched too loosely — a regression that
swallowed the codec refusal as some other error type would have passed
the test. The actual surface error is `RuntimeError` (the
`parse_expr` → `PyRuntimeError` conversion in `expr.rs::from_bytes`).
Switch to the concrete type and add a brief comment explaining why.

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

Author: timsaucer

python/tests/test_pickle_expr.py

@@ -294,7 +294,11 @@ def test_strict_decoder_refuses_inline_payload(self):
 
         strict_receiver = SessionContext().with_python_udf_inlining(False)
         strict_receiver.register_udf(u)
-        with pytest.raises(Exception, match="inlining is disabled"):
+        # `RuntimeError` (not bare `Exception`): the codec refusal is
+        # surfaced through `parse_expr` → `PyRuntimeError`. Tightening
+        # the assertion catches a regression that swallows the refusal
+        # as a different error type.
+        with pytest.raises(RuntimeError, match="inlining is disabled"):
             Expr.from_bytes(blob, ctx=strict_receiver)
 
     def test_sender_ctx_propagates_through_pickle(self):