fix(langchain_v1): export ModelResponse from agents.middleware (#33453) (#33454)

## Description

  Fixes #33453

`ModelResponse` was defined in `types.py` and included in its `__all__`
list, but was not exported from the middleware package's `__init__.py`.
This caused `ImportError` when attempting to import it directly
from `langchain.agents.middleware`, despite being documented as a public
export.

  ## Changes

- Added `ModelResponse` to the import statement in
`langchain/agents/middleware/__init__.py`
- Added `ModelResponse` to the `__all__` list in
`langchain/agents/middleware/__init__.py`
- Added comprehensive unit tests in `test_imports.py` to verify the
import works correctly

  ## Issue

  The original issue reported that the following import failed:

  ```python
  from langchain.agents.middleware import ModelResponse
# ImportError: cannot import name 'ModelResponse' from
'langchain.agents.middleware'

  The workaround was to import from the submodule:

from langchain.agents.middleware.types import ModelResponse # Workaround

  Solution

  After this fix, ModelResponse can be imported directly as documented:

  from langchain.agents.middleware import ModelResponse  # Now works!

  Testing

- ✅ Added 3 unit tests in
tests/unit_tests/agents/middleware/test_imports.py
  - ✅ All tests pass locally: make format, make lint, make test
  - ✅ Verified ModelResponse is properly exported and importable
  - ✅ Verified ModelResponse appears in __all__ list

  Dependencies

  None. This is a simple export fix with no new dependencies.

---------

Co-authored-by: Eugene Yurtsev <eugene@langchain.dev>
Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>

libs/langchain_v1/langchain/agents/middleware/__init__.py

@@ -20,6 +20,7 @@ from .types import (
     AgentMiddleware,
     AgentState,
     ModelRequest,
+    ModelResponse,
     after_agent,
     after_model,
     before_agent,
@@ -41,6 +42,7 @@ __all__ = [
     "ModelCallLimitMiddleware",
     "ModelFallbackMiddleware",
     "ModelRequest",
+    "ModelResponse",
     "PIIDetectionError",
     "PIIMiddleware",
     "PlanningMiddleware",