feat: Add LangChain middleware integration

[dagardner-nv]

Overview

• Adds a LangChain integration package located in python/nemo_flow/integrations/langchain
• Adds a nemo_flow.utils.run_sync method, I figured other integrations will likely need this.
• Defined nemo_flow as known-first party in the ruff settings, this triggered several formatting changes beyond the scope of this PR
• Work-around for wasm-pack GitHub repo migration causing the package to fail to be installed with taiki-e/install-action

• I confirm this contribution is my own work, or I have the right to submit it under this project's license.
• I searched existing issues and open pull requests, and this does not duplicate existing work.

Details

Where should the reviewer start?

Related Issues: (use one of the action keywords Closes / Fixes / Resolves / Relates to)

• Closes #

Summary by CodeRabbit

• New Features

  • LangChain integration: middleware and callbacks to route model and tool calls through agents
  • Sync-run utility to execute async coroutines from sync code while preserving context

• Documentation

  • Integration guide and usage examples added; integrations overview restructured

• Tests

  • New unit and integration tests for LangChain integration and the sync-run utility

• Chores

  • CI updated to run Python integration tests, cache integration deps, build integration wheels, and ignore editor/third-party paths

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Walkthrough

This pull request introduces a complete LangChain integration for NeMo Flow. It adds a new Python package (langchain-nemo-flow) with serialization utilities, a callback handler bridging LangChain chain lifecycle events to NeMo Flow scopes, a middleware routing model and tool calls through NeMo Flow, a sync/async execution bridge utility, comprehensive test coverage, CI/CD wiring, and documentation updates.

Changes

LangChain NeMo Flow Integration

Layer / File(s)	Summary
Package & Build Configuration integrations/langchain/pyproject.toml	Declares langchain-nemo-flow package with dynamic SCM versioning, runtime dependencies (langchain >=1.2.11,<2.0.0, langchain-core, nemo-flow), test group with pytest, and tooling config (ruff, uv).
Package Initialization integrations/langchain/src/langchain_nemo_flow/__init__.py	Establishes package surface; exports NemoFlowCallbackHandler and NemoFlowMiddleware via __all__.
Serialization & Codec Inference integrations/langchain/src/langchain_nemo_flow/_serialization.py	Implements LangChain↔NeMo Flow serialization: model-name extraction, codec inference (Anthropic vs OpenAI with use_responses_api support), ModelRequest↔payload conversion with system-message splitting, ModelResponse JSON wrapping under dedicated key with fallback decoding.
Callback Handler integrations/langchain/src/langchain_nemo_flow/callbacks.py	Implements NemoFlowCallbackHandler subclassing BaseCallbackHandler to bridge LangChain chain lifecycle (on_chain_start/on_chain_end/on_chain_error) to NeMo Flow scope push/pop with run-id, parent-id, metadata tracking, and error suppression.
Middleware & Call Routing integrations/langchain/src/langchain_nemo_flow/middleware.py	Implements NemoFlowMiddleware routing sync/async model calls (via wrap_model_call/awrap_model_call) and tool calls (via wrap_tool_call/awrap_tool_call) through NeMo Flow; prepares LLMRequest, infers codec, executes, and converts responses; forwards NeMo-overridden tool arguments via request.override(...).
Sync/Async Bridge Utility python/nemo_flow/utils.py	Adds run_sync(coro) function to execute awaitables from sync contexts: detects running event loop, uses asyncio.run() when none, offloads to ThreadPoolExecutor with contextvars and NeMo scope-stack propagation when loop is running.
Callback Handler Tests integrations/langchain/tests/test_callbacks.py	Unit tests for NemoFlowCallbackHandler validating scope lifecycle, metadata/naming, parent-child wiring, graceful no-ops when nemo_flow unavailable, and error suppression.
Middleware Tests integrations/langchain/tests/test_middleware.py	Tests NemoFlowMiddleware via RecordingMiddleware subclass verifying sync/async model-call routing, request mutation propagation, and correct return values; validates infer_codec_from_model codec selection for Anthropic/OpenAI with use_responses_api support.
Run Sync Concurrency Tests python/tests/test_utils.py	Verifies run_sync supports concurrent calls from multiple threads with running event loops and preserves NeMo Flow scope-stack identity.
CI/CD Infrastructure .github/ci-path-filters.yml, .github/workflows/ci_python.yml, justfile	Adds integrations/langchain/** to Python path filter; caches integrations/langchain/uv.lock in Test/Package jobs; adds test-python-integration justfile target; runs integration tests in CI Test job; builds LangChain wheel in Package job (amd64); integrates into test-all.
Type Checking & Documentation pyproject.toml, .gitignore, integrations/langchain/README.md, AGENTS.md, README.md, python/nemo_flow/__init__.py	Expands type-check allowed-unresolved-imports for langchain/langgraph patterns; adds .gitignore entries for .vscode and third_party/deepagents/; introduces integrations/langchain README with setup and usage examples; documents public API-based integrations in project overview; documents intentional non-import of utils.py to avoid executor creation.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

🚥 Pre-merge checks | ✅ 3 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 38.60% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.
Description check	⚠️ Warning	PR description is incomplete. Required sections 'Details' and 'Where should the reviewer start?' are empty or contain only template comments.	Complete the 'Details' section with a comprehensive summary of changes and fill in 'Where should the reviewer start?' to guide reviewers to critical files or decisions.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title follows Conventional Commits format with type 'feat' and provides a concise, imperative summary of the main change under 72 characters.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[Salonijain27]

Approved from a dependency point of view

[Salonijain27]

Approved from a dependency point of view

[dagardner-nv]

/merge