feat(conformance): Replaces invocation_id with user_message_index in the Recording

`invocation_id` is per-request, so we just need user_message_index to help locate where to start replay.

PiperOrigin-RevId: 807300016

src/google/adk/cli/conformance/test_case.py

@@ -81,8 +81,8 @@ class Recording(BaseModel):
       extra="forbid",
   )
 
-  invocation_id: str
-  """Unique invocation identifier."""
+  user_message_index: int
+  """Index of the user message this recording belongs to (0-based)."""
 
   agent_name: str
   """Name of the agent."""