refactor(@angular/build): extract shared breadth-first initial file analysis

[clydin]

Previously, both BundlerContext and optimizeChunks maintained separate implementations for discovering transitive initial files and calculating their import depths from the esbuild metafile. Both implementations used a depth-first stack traversal via Array.prototype.pop(), which visited deeper branches in reverse import order and failed to propagate updated minimum depths to the transitive dependencies of already-visited chunks in diamond dependency graphs.

This commit extracts a shared findTransitiveInitialFiles helper that uses a breadth-first queue traversal over direct static imports. Processing chunks in breadth-first order guarantees that each file is discovered at its true shortest import depth on first visit, preserves the natural declaration order of imports at each depth level, and eliminates the duplicated metafile traversal logic across initial bundling and chunk optimization.

[gemini-code-assist]

Code Review

This pull request refactors the transitive initial files analysis by centralizing the logic into a new helper function, findTransitiveInitialFiles, in bundler-files.ts and adding comprehensive unit tests. The chunk optimizer and bundler context have been updated to use this new helper. Feedback on the changes suggests replacing the for...of loop in findTransitiveInitialFiles with a standard index-based for loop to avoid potential issues when mutating the queue array during iteration.