Cleanup terminal reporter exception flattening by removing FlatException DTO

[Copilot]

✨ Enhancement

The terminal reporter still carried a local FlatException wrapper that added indirection without adding behavior. This change simplifies exception shaping by removing that type and using a single shared value shape end-to-end.

• What does this improve?

  • Removes the terminal-specific FlatException record from the exception flattening path.
  • Standardizes flattened exception data as named tuples (ErrorMessage, ErrorType, StackTrace) across terminal rendering and dotnet-test IPC conversion.
  • Reduces type surface area in reporter internals while preserving output semantics.

• Why is this valuable?

  • Eliminates a redundant transport type in a hot formatting path.
  • Makes exception-flow code easier to follow by keeping the flattened payload structural and lightweight.
  • Lowers maintenance overhead when evolving error formatting/forwarding logic.

• Implementation approach:

  • Updated ExceptionFlattener.Flatten(...) to return:

    (string? ErrorMessage, string? ErrorType, string? StackTrace)[]

  • Removed FlatException and updated all consumers:
    • Terminal test completion/rendering methods
    • Terminal formatting helpers (FormatErrorMessage, FormatStackTrace, inner exception formatting)
    • DotnetTest IPC exception mapping
    • Unit tests that assert flatten ordering/content

Example (new shape):

(string? ErrorMessage, string? ErrorType, string? StackTrace)[] exceptions =
    ExceptionFlattener.Flatten(errorMessage, exception);

[Evangelink]

It doesn't seem to be simplifying code, I'd say the opposite so closing

[Copilot]

Pull request overview

This PR removes the terminal reporter’s FlatException DTO and standardizes flattened exception data as a named tuple array (ErrorMessage, ErrorType, StackTrace) across terminal rendering, dotnet-test IPC conversion, and the associated unit tests.

Changes:

• Updated ExceptionFlattener.Flatten(...) to return an array of named tuples instead of FlatException[].
• Updated terminal reporter formatting + test completion paths to consume the tuple shape directly.
• Updated dotnet-test IPC exception mapping and unit tests to use the new flattened exception shape.

Show a summary per file

File	Description
test/UnitTests/Microsoft.Testing.Platform.UnitTests/OutputDevice/Terminal/TerminalTestReporterTests.cs	Updates tests to assert against tuple-shaped flattened exceptions.
src/Platform/Microsoft.Testing.Platform/ServerMode/DotnetTest/IPC/DotnetTestDataConsumer.cs	Adapts IPC exception mapping to consume the tuple-shaped flattening output.
src/Platform/Microsoft.Testing.Platform/OutputDevice/Terminal/TerminalTestReporter.TestCompletion.cs	Threads tuple-shaped flattened exceptions through test completion/rendering pipeline.
src/Platform/Microsoft.Testing.Platform/OutputDevice/Terminal/TerminalTestReporter.Formatting.cs	Updates formatting helpers to accept and read tuple-shaped flattened exception data.
src/Platform/Microsoft.Testing.Platform/OutputDevice/Terminal/ExceptionFlattener.cs	Removes FlatException record and emits named tuples directly from Flatten(...).

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 0