feat(langchain): add a modelRetryMiddleware

[christian-bromann]

This PR introduces modelRetryMiddleware, a new agent middleware that provides granular retry control for model calls within agents. This middleware leverages the extended RunnableRetry functionality to offer production-ready retry capabilities with configurable backoff, exception filtering, and failure handling strategies.

Motivation

Previously, applying retry logic to models used in createAgent was challenging:

• Using .withRetry() directly on models changed their type, causing incompatibility with createAgent
• No way to configure retry behavior at the agent middleware level
• Limited control over which exceptions trigger retries and how failures are handled

This middleware solves these issues by providing retry logic at the middleware level, allowing fine-grained control without type incompatibilities.

Features

Retry Configuration Options

• maxRetries: Maximum number of retry attempts after the initial call (default: 2)
• retryOn: Filter which exceptions trigger retries (function or array of error constructors)
• onFailure: Control failure behavior:
  • "raise" (default): Re-raise the exception
  • "return_message": Return an AIMessage with error details
  • Custom function: Format error message as needed
• backoffFactor: Exponential backoff multiplier (default: 2.0)
• initialDelayMs: Initial delay before first retry (default: 1000ms)
• maxDelayMs: Maximum delay cap (default: 60000ms)
• jitter: Enable random jitter to avoid thundering herd (default: true)

Implementation Details

• Reuses RunnableRetry: Leverages the extended retry logic from RunnableRetry.withRetry() by applying retry options directly to the model instance (see feat(core): extended retry logic for RunnableRetry #9446)
• Middleware Integration: Seamlessly integrates with the agent middleware system via wrapModelCall hook
• Type Safe: Works with create_agent without type incompatibilities
• Error Handling: Provides flexible failure handling strategies beyond simple exception raising

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