fix: forward reference resolution in Pydantic schema generation.

FUTURE_COPYBARA_INTEGRATE_REVIEW=#6099 from googleapis:release-please--branches--main 4597d87
PiperOrigin-RevId: 829577742

Author: shawn-yang-google

vertexai/_genai/_agent_engines_utils.py

@@ -590,6 +590,11 @@ def _is_pydantic_serializable(param: inspect.Parameter) -> bool:
     if param.annotation == inspect.Parameter.empty:
         return True
 
+    # Forward references can't be resolved by Pydantic if the type is not
+    # available at runtime (e.g. inside `if TYPE_CHECKING:`).
+    if "ForwardRef" in repr(param.annotation):
+        return False
+
     if isinstance(param.annotation, str):
         return False