perf(@angular/build): optimize library dts bundling and enable dts tree-shaking

[alan-agius4]

Summary

• Enable .d.ts tree-shaking: Set treeshake: isDts ? { moduleSideEffects: false } : false in executeMultiBundle to strip unexported internal types and unused namespace declarations from .d.ts bundles.
• Bypass dts() wrapper in bundleAllDts: Cache the import('rolldown-plugin-dts') promise at module scope and invoke createFakeJsPlugin directly instead of calling dts({ dtsInput: true, ... }) and filtering out rolldown-plugin-dts:resolver. This avoids unconditionally instantiating a native oxc-resolver (ResolverFactory) on every build.
• Disable Rolldown tsconfig auto-discovery and plugin timings: Pass tsconfig: false and checks: { circularDependency: false, pluginTimings: false } to rolldown() in executeMultiBundle.

Benchmark

Metric	main	Branch	Delta
Runs (ms)	12086, 11842, 12025, 12057, 11634	11947, 11726, 11606, 11760, 11868
Median (ms)	12025 ms	11760 ms	-2.20%
Mean (ms)	11929 ms	11781 ms	-1.24%
Min (ms)	11634 ms	11606 ms	-0.24%

[gemini-code-assist]

Code Review

This pull request optimizes the library bundler pipeline by dynamically loading rolldown-plugin-dts and directly instantiating createFakeJsPlugin to avoid unnecessary native resolver instantiation. It also introduces logic to skip empty or virtual-only .d.ts chunks (such as rolldown-runtime helpers) and adds corresponding unit and integration tests. Feedback was provided to ensure that skipped .d.ts chunks are not actively imported by other chunks, which could otherwise lead to broken import references. A code suggestion was provided to track and exclude imported files from being skipped.