feat(mcp-adapters): checkpoint elicitation with LangGraph interrupts

[Baihao Wang (byhow)]

What changes?

Modern MCP tools pause checkpointed LangGraph runs for form or URL input by default. Each invocation has its own interrupt scope, including parallel calls to the same tool. Resume answers must match the pending question keys and schemas; invalid answers reissue the questions with a validation message so callers can correct them on the same thread. The public interrupt exposes questions; effective arguments and opaque server continuation data stay in checkpointed request rounds, with automatic task retries disabled.

Graph/direct execution is selected when the tool is invoked, so tools discovered outside a graph can later run inside one. Modern requests advertise form and URL elicitation support. Graph calls require connection-level headers; direct calls retain per-call overrides. Legacy servers keep their configured callbacks.

How was it tested?

• 401 tests passed after merging feat(mcp-adapters): handle direct tool continuations #11643's review fixes, with two existing skips. Package/example type checks, ESM/CommonJS builds, attw, publint, lint and formatting passed.
• Real local servers cover reconstructed adapters, multi-round form/URL input, invalid answers, accept/decline/cancel, state-only responses, cancellation and round limits.
• Hook and capability checks cover invocation-time graph selection, retained arguments, rejected graph header overrides and automatic legacy negotiation.
• Regression tests first reproduced swapped answers between parallel calls and threads stuck after malformed answers, then passed with invocation isolation and answer correction. Initial server calls and hooks are not repeated.

Scope and limits

Third replacement slice of #11615, stacked on #11643. Detailed interrupt/resume guidance is preserved in the separate langchain-ai/docs draft. No OAuth additions, dependency or CI changes.

Revision-specific public SDK elicitation validators remain a prerequisite for the major release. The SDK continuation and request-metadata overrides remain maintenance dependencies. A crash between a server action and checkpoint persistence can repeat that action; checkpointing does not guarantee exactly-once execution. This integration covers tool elicitation, not sampling or prompt/resource interruption.

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@langchain/mcp-adapters	Minor

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