Fix type mapping and improve error handling across modules

[antosubash]

Summary

This PR addresses several issues across the codebase: improves TypeScript type generation for nullable value types and dictionaries, enhances file storage deletion error handling, adds file upload validation, and fixes a React key warning.

Key Changes

TypeScript Type Generation (TypeMappingHelpers.cs)

• Fixed mapping of nullable value types (e.g., int?) by stripping nested global:: prefixes that Roslyn adds during compilation
• Added support for mapping Dictionary<K, V>, IDictionary<K, V>, and IReadOnlyDictionary<K, V> to TypeScript Record<K, V> type
• Added FindTopLevelComma() helper to correctly parse generic type arguments while respecting nested angle brackets
• Added mapping for object/System.Object to TypeScript unknown type
• Updated test expectations to reflect correct nullable value type mapping

File Storage Service (FileStorageService.cs)

• Improved DeleteFileAsync() to handle storage deletion failures gracefully
• Storage path is now captured before database deletion to ensure it's available for logging if deletion fails
• Added try-catch block around storage provider deletion with best-effort error handling
• Added new LogStorageDeletionFailed() logger method to track orphaned storage data

File Upload Endpoint (UploadEndpoint.cs)

• Added validation to reject requests with missing or empty files
• Changed IFormFile parameter to nullable and added explicit null/empty check
• Returns BadRequest with descriptive message for invalid uploads

Chart Component (chart.tsx)

• Fixed React key warning by using composite key combining parent key and item value instead of item value alone

Notable Implementation Details

• The FindTopLevelComma() method uses a depth counter to track angle bracket nesting, ensuring correct parsing of complex generic types like Dictionary<List<string>, int>
• Storage deletion failure is logged but doesn't throw, preventing cascading failures after successful database cleanup
• File validation happens before stream opening to fail fast on invalid requests