Fix Storage Account connection string handling to prevent app crashes

[Copilot]

The Azure Web App was crashing with System.InvalidOperationException when the Storage Account connection string was missing or empty. This PR fixes the issue by replacing unhandled exceptions with proper HTTP error responses and helpful guidance.

Changes Made

1. Improved Error Handling in AppController

• Before: Threw InvalidOperationException causing app crashes
• After: Returns proper HTTP status codes with detailed error messages

// Before: Unhandled exception
throw new InvalidOperationException("Storage account connection string is not configured.");

// After: Graceful HTTP response
return BadRequest(new { 
    error = "Storage account connection string is not configured.", 
    details = "Please configure the 'StorageAccount' connection string in app settings or appsettings.json",
    configurationKey = "ConnectionStrings:StorageAccount"
});

2. Added Development Configuration

• Added ConnectionStrings section to appsettings.Development.json with development storage placeholder
• Ensures consistent configuration structure across environments

Testing Results

✅ Missing connection string: Returns HTTP 400 with helpful guidance instead of crashing
✅ Invalid connection string: Returns HTTP 500 with connection details instead of unhandled exception
✅ Application builds and runs successfully
✅ No more System.InvalidOperationException crashes

Benefits

• Improved reliability: App no longer crashes on missing configuration
• Better debugging: Clear error messages help identify configuration issues quickly
• Enhanced developer experience: Proper HTTP status codes and detailed guidance
• Production readiness: Graceful handling of configuration errors

The app now handles missing storage account configuration gracefully and provides clear guidance to developers and operations teams for resolving configuration issues.

Fixes #57.

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