fix(langgraph): support falsy command resume values

[xiaotianna]

Summary

Allow falsy values such as false, 0, and an empty string to be used as
Command.resume values.

PregelLoop determines whether a resume value is present using
resume != null, but mapCommand previously used a truthiness check. As a
result, falsy resume values produced no pending writes and caused:

EmptyInputError: Received empty Command input

Changes

• Treat only null and undefined as missing resume values
• Preserve false, 0, and "" as valid resume values
• Add regression tests for these falsy values

Tests

pnpm exec vitest run src/pregel/io.mapCommand.test.ts --project unit --typecheck.enabled=false

Result: 14 tests passed.

[changeset-bot]

🦋 Changeset detected

Latest commit: 86baa79

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@langchain/langgraph	Patch

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commit: 86baa79

[xiaotianna]

This failure seems unrelated to this PR's code changes.

The failing workflow is .github/workflows/dependabot-auto-merge.yml, which uses:

dependabot/fetch-metadata@25dd0e34f4fe68f24cc83900b1fe3fe149efef98

The action is pinned to a full commit SHA, but dependabot/fetch-metadata@* is not included in the repository/enterprise allowed action patterns.

This PR only changes libs/langgraph-core/src/pregel/io.ts, its test file, and a changeset, so I don't think this CI failure is caused by my change.

Could a maintainer either add dependabot/fetch-metadata@* to the Actions allowlist, or update/disable the Dependabot auto-merge workflow?