refactor(cdk/schematics): remove TODO by centralizing stylesheet discovery
This PR refactors the stylesheet discovery logic within the CDK schematics to centralize and simplify how .css and .scss files are located.
It removes a previous TODO by introducing a shared utility that ensures consistent behavior across all schematics relying on global or project-level styles.
Closes #24032
Deployed dev-app for 0234edc5e8a764c175db27970bde69efce5c3ed3 to: https://ng-dev-previews-comp--pr-angular-components-32281-dev-q4fpof7p.web.app
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Deployed docs-preview for 0234edc5e8a764c175db27970bde69efce5c3ed3 to: https://ng-dev-previews-comp--pr-angular-components-32281-docs-12rxqgv9.web.app
Note: As new commits are pushed to this pull request, this link is updated after the preview is rebuilt.