Remove FlatException type from terminal test reporter

[Copilot]

Plan: Cleanup Terminal Test Reporter by Removing FlatException Type ✓ COMPLETE

Based on the issue and code analysis, the goal was to optimize the terminal test reporter by eliminating the FlatException type.

Changes completed:

• Analyze current usage of FlatException in TerminalTestReporter
• Analyze current usage of FlatException in DotnetTestDataConsumer
• Design replacement approach that works directly with Exception objects
• Update ExceptionFlattener to return tuple (CustomErrorMessage, Exception[]) instead of FlatException[]
• Update TerminalTestReporter to work with Exception arrays
• Update DotnetTestDataConsumer to work with Exception arrays
• Remove FlatException record type
• Fix bug where inner exceptions were using 'aggregate' instead of 'currentException'
• Fix List type to use Exception? for nullable support
• Improve variable naming for clarity
• Validate logic with comprehensive tests
• Code review completed and addressed
• Security checks passed (CodeQL)
• Revert unintended .gitignore changes

Key improvements:

1. Eliminated intermediate allocation: No longer creating FlatException objects - reduces memory overhead
2. Fixed original bug: Inner exception loop now correctly uses currentException instead of aggregate
3. Cleaner API: Returns tuple with custom error message and exceptions array
4. Direct property access: Exception properties accessed directly when rendering
5. Better null handling: List properly typed as Exception? to handle nullable inner exceptions
6. Improved readability: Better variable names clarify intent

Test Results:

All test scenarios verified and passing:

• ✓ Exception with custom error message
• ✓ Exception without custom error message
• ✓ Exceptions with inner exceptions (chain)
• ✓ AggregateException with multiple inner exceptions
• ✓ Error message without exception
• ✓ Whitespace handling
• ✓ Simple exception with no inner exception

Files Changed:

• ExceptionFlattener.cs - Refactored to return tuple, removed FlatException type
• TerminalTestReporter.cs - Updated to work with Exception arrays
• DotnetTestDataConsumer.cs - Adapted to new exception handling

Addresses issue: #6031

Ready for review!

Original prompt

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This section details on the original issue you should resolve

<issue_title>Cleanup terminal test reporter</issue_title>
<issue_description>yes, there are probably more optimizations that can be done in the terminal logger, e.g. getting rid of the flatException type (if you did not do that already)

Originally posted by @nohwnd in #6025 (comment)
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Cleanup terminal test reporter #6031

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