feat(langchain_v1): Implement PIIMiddleware (#33271)

- supports 6 well-known PII types (email, credit_card, ip, mac_address,
url)
- 4 handling strategies (block, redact, mask, hash)
- supports custom PII types with detector functions or regex
- the built-in types were chosen because they are common, and detection
can be reliably implemented with stdlib

Author: nfcampos

.claude/settings.local.json

@@ -3,6 +3,11 @@
     "allow": [
       "Bash(uv run:*)",
       "Bash(make:*)"
+      "WebSearch",
+      "WebFetch(domain:ai.pydantic.dev)",
+      "WebFetch(domain:openai.github.io)",
+      "Bash(uv run:*)",
+      "Bash(python3:*)"
     ],
     "deny": [],
     "ask": []

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

@@ -1,6 +1,7 @@
 """Middleware plugins for agents."""
 
 from .human_in_the_loop import HumanInTheLoopMiddleware
+from .pii import PIIDetectionError, PIIMiddleware
 from .planning import PlanningMiddleware
 from .prompt_caching import AnthropicPromptCachingMiddleware
 from .summarization import SummarizationMiddleware
@@ -23,6 +24,8 @@
     "AnthropicPromptCachingMiddleware",
     "HumanInTheLoopMiddleware",
     "ModelRequest",
+    "PIIDetectionError",
+    "PIIMiddleware",
     "PlanningMiddleware",
     "SummarizationMiddleware",
     "ToolCallLimitMiddleware",