fix(internal): align build setup with LC

[christian-bromann]

This PR aligns LangGraph.js's build process with the patterns established in LangChain.js by migrating from a custom build CLI to tsdown with standardized configuration.

Key Changes

• Migrate build system to tsdown: Replace the custom @langchain/build CLI with tsdown-based builds using a shared getBuildConfig() helper
• Standardize package builds: Each package now has its own tsdown.config.ts that uses shared configuration defaults
• Add build-time validation: Integrate ATTW (Are The Types Wrong), publint, and unused dependency checking into the build pipeline
• Update Turbo pipeline: Rename build:internal → build:compile and update task dependencies
• CJS compatibility plugin: Add cjsCompatPlugin to handle CommonJS barrel file generation for better backwards compatibility

Build Configuration

Each package's tsdown.config.ts now follows a standard pattern:

import { getBuildConfig, cjsCompatPlugin } from "@langchain/build";

export default getBuildConfig({
  entry: ["./src/index.ts"],
  plugins: [
    cjsCompatPlugin({
      files: ["dist/", "CHANGELOG.md", "README.md", "LICENSE"],
    }),
  ],
});

The shared config provides:

• Dual format output (ESM + CommonJS)
• TypeScript declaration files (.d.ts and .d.cts)
• Source maps
• Automatic exports field generation
• Build validation (ATTW, publint, unused deps)

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