feat(langchain): add HealthMiddleware for agent health monitoring and resilience

[ixchio]

Add built-in health-aware middleware for LangChain v1 agents that monitors tool/model health and provides automatic retry, fallback, and telemetry capabilities.

Fixes #34193

Features

• HealthStats: Rolling window statistics tracking per-tool/model (error rate, P95 latency)
• HealthPolicy: Configurable degradation thresholds
• HealthMiddleware: Auto retry with exponential backoff, model fallback, tool disabling
• Telemetry: Pluggable emitter with create_langsmith_emitter() helper
• Persistence: Optional store integration for cross-run stats via store param
• Provider tool preservation: Keep provider dict tools active even when degraded

Usage

from langchain.agents.middleware import HealthMiddleware, HealthPolicy

health = HealthMiddleware(
    policy=HealthPolicy(max_error_rate=0.2, consecutive_failures=3),
    fallback_model="openai:gpt-4o-mini",
    disable_tools=True,
)

agent = create_agent(model="openai:gpt-4o", tools=[...], middleware=[health])

Testing
35 unit tests covering HealthStats, HealthPolicy, HealthMiddleware
All 534 middleware tests pass