perf(@angular/build): decouple diagnostic type checking from build start

[alan-agius4]

Previously, the compiler plugin sequentially awaited compilation.diagnoseFiles() in build.onStart before returning to esbuild.

Because build.onStart blocked until TypeScript diagnostic checks finished, esbuild's Go bundler sat idle while single-threaded type checking occurred, preventing module resolution and bundling from running concurrently.

To overlap bundling with type checking:

• Initiate compilation.diagnoseFiles() asynchronously during build.onStart without awaiting its completion.
• Allow build.onStart to return immediately once compilation emit is complete, enabling esbuild to begin bundling and file resolution concurrently.
• Await the diagnostics promise in build.onEnd, merging any diagnostic errors or warnings into the final build result.

In benchmarks on clean builds, this overlaps diagnostic checks with bundling, saving ~150 ms in small projects and 500 ms to 2,000+ ms in large enterprise codebases.

[gemini-code-assist]

Code Review

This pull request defers the resolution of Angular compilation diagnostics by storing the diagnosticsPromise during the start phase and awaiting it asynchronously inside the build.onEnd callback. Feedback on these changes highlights two issues: first, a potential unhandledRejection in Node.js if compilation.diagnoseFiles rejects before onEnd is called, which can be mitigated by attaching a .catch() handler immediately; second, in watch mode, hasCompilationErrors is not reset to true at the start of a rebuild, which can incorrectly signal to other compilations that there are no errors if a rebuild fails early.