`ios_app_clip` should validate that `application-identifier` is set in entitlements

Xcode automatically sets `application-identifier` in an App Clip's entitlements. If it's not set, then app clips fail to install because `(null)` is not a prefix of whatever `com.apple.developer.parent-application-identifiers` is set to. Ideally it should validate that `com.apple.developer.parent-application-identifiers` is set as well, and that `application-identifier` is a prefix of it.

Start at the ios_app_clip rule and its entitlements validation entry point; the issue does not name a file or test. Trace how application-identifier and com.apple.developer.parent-application-identifiers are read, then verify that missing values and an invalid prefix are rejected with focused coverage.

Written by the indexing model from the issue text.