fix(langchain): re-export PIIMatch from middleware package (#39291)

Closes #38718

`PIIMatch` was already in `pii.py`'s `__all__`, but not re-exported from
the package `__init__`, forcing custom PII detector authors to import it
from the private `_redaction` module.


---------

Co-authored-by: @abhi-0203 <abhi-0203@users.noreply.github.com>

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

@@ -11,7 +11,7 @@ from langchain.agents.middleware.human_in_the_loop import (
 from langchain.agents.middleware.model_call_limit import ModelCallLimitMiddleware
 from langchain.agents.middleware.model_fallback import ModelFallbackMiddleware
 from langchain.agents.middleware.model_retry import ModelRetryMiddleware
-from langchain.agents.middleware.pii import PIIDetectionError, PIIMiddleware
+from langchain.agents.middleware.pii import PIIDetectionError, PIIMatch, PIIMiddleware
 from langchain.agents.middleware.provider_tool_search import ProviderToolSearchMiddleware
 from langchain.agents.middleware.shell_tool import (
     CodexSandboxExecutionPolicy,
@@ -70,6 +70,7 @@ __all__ = [
     "ModelRetryMiddleware",
     "OutputAgentState",
     "PIIDetectionError",
+    "PIIMatch",
     "PIIMiddleware",
     "ProviderToolSearchMiddleware",
     "RedactionRule",

libs/langchain_v1/tests/unit_tests/agents/middleware/implementations/test_pii.py

@@ -19,7 +19,9 @@ from langgraph.stream._types import ProtocolEvent
 from langgraph.stream.transformers import MessagesTransformer
 
 from langchain.agents import AgentState
+from langchain.agents import middleware as middleware_package
 from langchain.agents.factory import create_agent
+from langchain.agents.middleware import PIIMatch as PublicPIIMatch
 from langchain.agents.middleware._redaction import RedactionRule
 from langchain.agents.middleware.pii import (
     PIIDetectionError,
@@ -34,6 +36,24 @@ from langchain.agents.middleware.pii import (
 )
 from tests.unit_tests.agents.model import FakeToolCallingModel
 
+# ============================================================================
+# Public Export Tests
+# ============================================================================
+
+
+class TestPIIMatchPublicExport:
+    """Test that `PIIMatch` is importable from the public middleware package.
+
+    Regression test: `PIIMatch` was only exported from the private
+    `_redaction` module, forcing custom-detector authors to import from it
+    directly instead of `langchain.agents.middleware`.
+    """
+
+    def test_pii_match_importable_from_middleware_package(self) -> None:
+        assert PublicPIIMatch is PIIMatch
+        assert "PIIMatch" in middleware_package.__all__
+
+
 # ============================================================================
 # Detection Function Tests
 # ============================================================================