[APPS] Refactor backend discovery AST parsing

[sdkennedy2]

Motivation

This is the first PR in the regenerated backend connection ID analysis stack. The existing backend discovery module mixes the core BackendFunction data model, ESTree/Rollup AST type narrowing, export discovery, and callable export validation in one file. Splitting those concerns first gives the later connection ID extraction PRs a stable AST parsing home and keeps their reviews focused on behavior changes instead of file movement.

Related planning context: plan 014, created after build-plugins#338 established the backend manifest upload plumbing.

Changes

This PR is intended to be behavior-neutral. It moves the core backend function datatype into backend/types.ts and moves the existing AST export discovery implementation into backend/ast-parsing/extract-backend-functions.ts.

The old backend/discovery.ts barrel was removed in response to review feedback. Existing callers now import from the owning modules directly: core backend types from backend/types.ts, and AST discovery helpers from backend/ast-parsing/*.

enumerateBackendExports(ast, filePath) now returns a stricter discriminated union instead of optional metadata fields:

type BackendExport = BackendLocalExport | BackendReExport;

interface BackendLocalExport {
    kind: 'local';
    name: string;
    localName: string;
}

interface BackendReExport {
    kind: 're-export';
    name: string;
    localName: string;
    source: string;
}

extractExportedFunctions(ast, filePath) continues to return the same string array as before. Existing validation semantics for unsupported backend export shapes are preserved.

QA Instructions

Focused tests:

NODE_OPTIONS=--experimental-vm-modules ~/.yarn/switch/bin/yarn workspace @dd/tests jest --config jest.config.ts ../plugins/apps/src/backend/discovery.test.ts --runInBand
~/.yarn/switch/bin/yarn workspace @dd/apps-plugin typecheck
~/.yarn/switch/bin/yarn lint
~/.yarn/switch/bin/yarn workspace @dd/tests test:e2e src/e2e/appsPlugin/appsPlugin.spec.ts

Manual linked-app verification against /Users/scott.kennedy/dd/test-action-catalog-app/:

~/.yarn/switch/bin/yarn cli prepare-link
~/.yarn/switch/bin/yarn workspace @datadog/vite-plugin build
DATADOG_API_KEY=fake DATADOG_APP_KEY=fake npm run build
DATADOG_API_KEY=fake DATADOG_APP_KEY=fake npx vite --host 127.0.0.1 --port 5178

The linked test app discovered and proxied all three imported backend functions, built all three backend bundles, produced the dry-run archive, and served generated backend proxy code in dev mode. prepare-link --revert was run afterward.

Blast Radius

This PR affects High Code Apps backend function discovery in @dd/apps-plugin. It should not change runtime behavior, manifest shape, dev preview payloads, backend proxy generation, or backend bundling. The main risk is an accidental import/export wiring regression from the module split, covered by the focused tests and apps-plugin E2E suite.

Documentation

• How to reliably make frontend production changes
• PR Description Guidelines
• Deploy steps

[sdkennedy2]

• [APPS] Extract inline backend connection IDs #346
• [APPS] Refactor backend discovery AST parsing #345 👈 (View in Graphite)
• master

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[sdkennedy2]

@codex review
@cursor review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Already looking forward to the next diff.

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[yoannmoinet]

Just some nit, not blocking for them.

[yoannmoinet]

Nit: Not a fan of barrel files in general, unless there is a very good reason for it. In this occurrence, the reason isn't particularly good IMO.
This usually leads to confusion, misdirection and build complexity.

[sdkennedy2]

I was thinking it could reduce the number of changes in the initial PR. My plan was to remove the reference in the next PR but you're right the total number of references to this file are now very small. I'll remove this file 👍

[yoannmoinet]

Nit: Using optionals is often revealing some smells 😃
Can't we have this stricter?

[sdkennedy2]

Backend functions are either defined within the *.backend.ts or they are imported then exported. I could probably break this down into a discriminated type union for the two cases.