docs(plugin): add experimental metadata to payloads

Mark execution_input and execution_result with
metadata={"experimental": True} and a leading EXPERIMENTAL docstring,
matching the pattern #625 established for OperationInfo.result /
OperationInfo.error / InvocationEndInfo.error.

Unlike a docstring alone the metadata is introspectable at runtime, so
the generic test #625 added -- which requires the marker on every
plugin info field named for input, output, result or error -- now
covers these two fields. That test fails on this branch without them.

This matters more since #625 removed the plugins= FutureWarning: the
per-field markers are now the only stability signal on these surfaces.

Refs #616

Author: Alex Wang

packages/aws-durable-execution-sdk-python/src/aws_durable_execution_sdk_python/plugin.py

@@ -176,12 +176,15 @@ class InvocationInfo:
     execution_arn: str | None
     is_first_invocation: bool
     execution_start_time: datetime.datetime | None = None
-    execution_input: Any = field(default=None, kw_only=True)
-    """EXPERIMENTAL: the deserialized execution input. May change or be removed.
+    execution_input: Any = field(
+        default=None,
+        kw_only=True,
+        metadata={"experimental": True},
+    )
+    """EXPERIMENTAL: The deserialized execution input, when available.
 
     Surfaced to instrumentation plugins that need to record it (e.g. Workflow
-    Insight). Mirrors the JS SDK's ``InvocationInfo.executionInput``. kw_only so
-    it never reorders the positional fields above.
+    Insight). Mirrors the JS SDK's ``InvocationInfo.executionInput``.
 
     Defaults to ``None`` only when the field is not populated (a hook info built
     without it); ``durable_execution()`` always populates it with the
@@ -202,13 +205,16 @@ class InvocationEndInfo(InvocationInfo):
         metadata={"experimental": True},
     )
     """EXPERIMENTAL: The invocation error, when available."""
-    execution_result: str | None = field(default=None, kw_only=True)
-    """EXPERIMENTAL: the serialized execution result. May change or be removed.
+    execution_result: str | None = field(
+        default=None,
+        kw_only=True,
+        metadata={"experimental": True},
+    )
+    """EXPERIMENTAL: The serialized execution result, when available.
 
     A JSON string, or ``""`` when the result was checkpointed out-of-band for a
-    large payload. Surfaced to instrumentation plugins that record execution
-    output (e.g. Workflow Insight); mirrors the JS SDK's
-    ``InvocationEndInfo.executionResult``. ``None`` on failure or suspend.
+    large payload. Mirrors the JS SDK's ``InvocationEndInfo.executionResult``.
+    ``None`` on failure or suspend.
     """
 
     @classmethod