feat(langchain): add Stagehand code-mode integration

[shrey150]

Summary

• add a LangChain Deep Agents harness that launches the local Stagehand code-mode MCP over stdio
• hold one explicit MCP session open for the complete agent invocation so browser state survives tool calls
• load the shared Stagehand V4 syntax guide into the agent system prompt
• constrain the MCP Python dependency to the compatible 1.x line
• add a real persistent-session discovery smoke and an isolated Deep Agents CI job

Stack

This PR is stacked on the CrewAI integration. Its diff contains only the LangChain Deep Agents integration.

Lifecycle

The harness opens client.session("stagehand"), loads tools from that session, and runs the complete agent invocation inside the same context. This explicit session is required because the convenience get_tools() path starts a new local MCP process for each tool call, which would discard browser state.

E2E Test Matrix

Command / flow	Observed output	Confidence / sufficiency
Build the local code-mode package, then run the Deep Agents smoke in an isolated Python 3.12 environment with the reviewed requirements	The built server reported that it was listening on stdio and the persistent LangChain session reported Deep Agents persistent stdio discovery PASS: code_execute.	Proves the real LangChain MCP adapter can launch the reviewed local stdio server, keep an explicit session open, discover exactly one tool, and receive the shared syntax guide. It does not make a provider-dependent model request.
Resolve the Python requirements from scratch	Resolution selected deepagents 0.7.4, langchain-mcp-adapters 0.3.1, and mcp 1.29.0; the isolated smoke then passed.	Proves the declared ranges form a compatible fresh environment and avoid the incompatible MCP 2.x API.
Targeted Prettier checks over the Deep Agents documentation and workflow	Completed without formatting differences.	Covers the changed Markdown and YAML surface; the executed Python smoke imports and runs every changed Python module.

[socket-security]

Review the following changes in direct dependencies. Learn more about Socket for GitHub.

Diff	Package	Supply Chain Security	Vulnerability	Quality	Maintenance	License
	pypi/​mcp@​1.29.0
	pypi/​langchain-mcp-adapters@​0.3.1

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