[APPS] Tree-shake backend function bundles

[ksun154]

What and why?

This is an alternative to #533 for the pitchGPT HTTP 413 failure ("bundle exceeds the maximum decompressed size of 52 MB").

Each exported backend function is bundled by its own vite.build(), from a virtual entry that imports only that one export. The shared backend config set rollupOptions.treeshake: false, so
Rollup kept every module the .backend.ts file imports, including code that only sibling exports use. A file with N exports therefore shipped N copies of its whole import graph. That is the
"14× for the largest file" multiplier in #533. Minifying makes each copy smaller but keeps all of them; tree-shaking removes the duplication.

I found no stated reason for treeshake: false. It arrived with the switch to separate vite.build() calls in e154acf, with no comment or commit-message rationale.

This PR only enables tree-shaking. It does not minify; minify: false is unchanged.

How?

• packages/plugins/apps/src/vite/build-config.ts: treeshake: false → treeshake: true. Production and dev-server builds share this config.
• Standard tree-shaking keeps each module's top-level side effects. It drops a module's import-time code only when the package declares "sideEffects": false, which is the same behavior as the
  frontend build.
• Static checks and connection-ID collection run in moduleParsed, which happens before tree-shaking. They still see every imported module, so allowlists and banned-import detection are
  unchanged.
• The injected setBackend / setExecuteActionImplementation calls run inside main(), so they are reachable and kept.

Testing

• New regression test in build-config.test.ts. It runs a real Vite build and checks that:
  • the used export and an imported module's import-time side effect are kept;
  • a helper used only by a sibling export, and an unused dependency export, are dropped.
• The test fails on master (the sibling-only code is present in the bundle) and passes with this change.
• packages/plugins/apps: 35 suites, 747 passed, 1 skipped. This includes the real @datadog/apps-backend SDK integration build and the local-execution suites.
• ESLint and tsc --noEmit are clean.

Measurements

Both builds have minification off.

	Total backend JS
Repo fixture (apps_backend_project, 7 functions), master	90,122 B
Repo fixture, this PR	71,271 B (−21%)
Synthetic file, 5 exports (zod / lodash-es / date-fns), master	8,920 KB
Synthetic file, this PR	284 KB (−97%)

The fixture has about one export per file, so it understates the gain for apps with many exports per file. pitchGPT has not been measured yet.

Risks and follow-ups

• Risk: a dependency that wrongly declares "sideEffects": false but relies on import-time code would lose that code. The frontend build has the same exposure. Fallback: treeshake: { moduleSideEffects: true }, at the cost of keeping more unused code.
• Minification is deferred, not rejected. It would still help, since the limit is on decompressed size, but it makes function-log stack traces hard to read (renamed locals, one long line, no server-side source maps known), lets production diverge from dev, and Proposal: smaller App Builder backend bundles #533's version adds a permanent apps.backend.minify option. A lighter variant (esbuild: { minifyIdentifiers: false }, untested) keeps local names readable.
• Next: merge this, measure pitchGPT, and add minification and Proposal: smaller App Builder backend bundles #533's decompressed-size warning only if the size is still close to the limit.

[datadog-datadog-prod-us1]

✅ All CI checks and tests passed.

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

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[Copilot]

Copilot review overview

🟡 Changes recommended

The new test violates the repository’s no-inlined-function-call-arguments standard.

Review effort: Balanced
Findings: 1

Open (1)

• Store serialized metadata before writing the file · New

What changed in this PR

Enables backend bundle tree-shaking to reduce duplicated code across separately bundled exports.

Changes:

• Enables Rollup tree-shaking for backend builds.
• Adds regression coverage for unused exports and import-time side effects.

File	Description
packages/​plugins/​apps/​src/​vite/​build-config.ts	Enables and documents tree-shaking.
packages/​plugins/​apps/​src/​vite/​build-config.test.ts	Tests retained side effects and removed unused code.

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[Copilot]

Copilot review overview

🟢 Approval recommended

The focused configuration change is supported by realistic regression coverage with no unresolved correctness issues.

Review effort: Balanced
Findings: None

Resolved since last review (1)

• Store serialized metadata before writing the file