Migrate SchemaDesignerAPI to JAX-RS V2 annotations with RESTful improvements

[Copilot]

• Analyse all mocks in DownloadConfigSetAPITest and GetConfigSetFileAPITest vs other API tests
• DownloadConfigSetAPITest: replace mock(ConfigSetService) + doAnswer/stubDownloadConfig helper with real FileSystemConfigSetService(createTempDir()) {} (anonymous subclass to access protected constructor); remove unused mock(SolrQueryRequest) and mock(SolrQueryResponse) (pass null instead); write real files with Files.createDirectories+Files.writeString; fix sanitization test assertion to check for sanitized filename string rather than absence of spaces (the Content-Disposition header itself has a space)
• GetConfigSetFileAPITest: same pattern — real FileSystemConfigSetService, no request/response mocks; null configset dir means not-found, missing file means not-found
• Fix GetConfigSetFile.downloadFileFromConfig(): handle null return from FileSystemConfigSetService (it silently returns null for missing files instead of throwing IOException) and throw a 404 SolrException — this was a real bug caught by switching from mocks to a real implementation
• Run gradlew tidy — passes
• Run both test classes — all 12 tests pass

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