Fix stale TypeScript SDK docs on bundle metadata and packing setup

[ColtenOuO]

Summary

airflow-core/docs/authoring-and-scheduling/language-sdks/typescript.rst still described NodeCoordinator's bundles_root as accepting a bundle.mjs "with embedded metadata, or with an airflow-metadata.yaml sidecar." That sidecar fallback was removed in #70273 (Remove airflow-metadata.yaml sidecar support from NodeCoordinator) — airflow-ts-pack always embeds the manifest in bundle.mjs itself, and NodeCoordinator now only reads that embedded metadata; no tool produces the sidecar file, and _bundle_metadata.py's YAML-sidecar parsing helpers are effectively dead code from the doc's point of view. The doc never got updated when the code changed, so it describes a deployment shape (a hand-written airflow-metadata.yaml next to the bundle) that Airflow no longer supports.

Separately, the "Building and packaging" section jumped straight to running npx airflow-ts-pack src/main.ts --outdir dist without ever showing how to get there: it never mentioned installing the apache-airflow-ts-sdk npm package, and it didn't mention that esbuild — the bundler airflow-ts-pack uses — is an optional peer dependency that the runtime install intentionally skips (see ts-sdk/package.json's peerDependenciesMeta.esbuild.optional). A user following the doc exactly as written would run npx airflow-ts-pack in a project that never had esbuild installed and hit a failure with no explanation in the doc of why, or what to install instead. The ts-sdk/README.md already has the correct sequence (npm install --save-dev esbuild before packing); the RST doc had drifted from it.

Change

• Removed the stale airflow-metadata.yaml sidecar mention from the NodeCoordinator configuration table; bundles_root now just documents the current, single supported shape (a bundle.mjs with embedded metadata).
• Added a "Prerequisites" bullet showing npm install apache-airflow-ts-sdk for authoring task handlers.
• Added an explanation plus npm install --save-dev esbuild to the "Building and packaging" section, before the airflow-ts-pack invocation, so the documented steps actually work end-to-end for a fresh project.

Docs-only change.

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Was generative AI tooling used to co-author this PR?

• Yes — Claude Code (Sonnet 5)

[ColtenOuO]

cc. @jason810496

[jason810496]

We intentionally keep the npm install ... command out of the airflow-core docs until the NPM package is ready.

Thanks for adding these back.