fix(@angular/build): resolve coverage overrides paths relative to workspace root

[maruthang]

Problem

When using @angular/build:karma, coverage overrides defined in karma.conf.js under coverageReporter.check.each.overrides (or global.overrides) are silently ignored.

Root cause

The builder sets karmaOptions.basePath to the temporary build output directory (e.g. dist/test-out/<uuid>). karma-coverage normalises each coverage-map key (an absolute source file path) by computing path.relative(basePath, absoluteKey). So /workspace/src/app/app.ts becomes ../../../src/app/app.ts, which never matches a user-written pattern like src/app/app.ts.

This was confirmed by a community contributor in #30956 who showed that the workaround '../../../src/app/app.ts' worked but was clearly not the intended API.

Fix

After the karma config is parsed, rewrite the keys of coverageReporter.check.global.overrides and coverageReporter.check.each.overrides so they are expressed as paths relative to basePath (the temp output path), which is exactly how karma-coverage normalises coverage-map keys.

This follows the same pattern already used for junitReporter.outputDir (adjusted post-parse to account for the changed basePath).

Relative patterns are resolved from workspaceRoot; absolute patterns are used as-is — both are then made relative to outputPath.

Behaviour change

Before	After
overrides: { 'src/app/app.ts': { ... } } silently ignored	pattern resolved from workspace root and correctly matched

Closes #30956

[gemini-code-assist]

Code Review

This pull request addresses an issue where Karma coverage reporter overrides were not being correctly applied due to path mismatches. The implementation now rebases override patterns from the workspace root to the Karma base path (output directory) within the application builder. Additionally, a regression test has been introduced to ensure that per-file coverage thresholds are properly respected during the test execution. I have no feedback to provide.