[APPS] Move inlineDynamicImports to shared backend build config

[sdkennedy2]

Motivation

Backend functions must each produce a single self-contained JS file — no shared chunks between them. Previously, buildBackendFunctions called viteBuild() once with all functions as multi-entry input. With multiple entries, Rollup can extract common code into shared chunks, which breaks the requirement of one standalone file per function.

Additionally, both the production build and dev server were independently setting inlineDynamicImports: true in their rollup output config, duplicating the intent of the shared base config.

Changes

• Build each function individually: viteBuild() is now called once per backend function instead of once for all functions. This ensures Rollup never sees multiple entries and cannot create shared chunks — each output is a single self-contained JS bundle.
• Centralize inlineDynamicImports: Moved inlineDynamicImports: true into getBaseBackendBuildConfig so the shared config is the single source of truth. Removed the redundant per-caller override in the dev server and stale comments referencing the now-centralized setting.

QA Instructions

• Existing unit tests for backend function building should pass (note: mockViteBuild is now expected to be called once per function instead of once total)
• E2E tests for apps with backend functions should produce identical output — each function is still a single self-contained JS file

Blast Radius

• Only affects the apps plugin's backend function build path (Vite only)
• No customer-facing API or config changes
• Gated behind the apps plugin feature flag

Documentation

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

[sdkennedy2]

• [APPS] Move inlineDynamicImports to shared backend build config #317 👈 (View in Graphite)
• [APPS] Add vite dev server middleware for backend functions #291 : 1 other dependent PR (#306 )
• [APPS] Use separate vite.build() for backend functions, drop Rollup backend support #292
• master

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[datadog-official]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

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