dotnet test: truncate large failing-host output in summary (#52297)

[Evangelink]

Fixes #52297

Problem

When a Microsoft.Testing.Platform test host exits with a failure, it often prints its entire command-line help (hundreds of lines — over 700 for xUnit) to standard output. The classic trigger is an invalid argument value, e.g. dotnet test --show-live-output true:

Option '--show-live-output' has invalid arguments: Invalid value 'true' (must be one of: 'on', 'off')
  Usage SharedAppHost.exe [option providers] [extension option providers]
...<700 more lines of help>...

dotnet test echoed all of it verbatim in the executable summary (AppendExecutableSummary), so the actual error — which appears at the top — was buried under a wall of noise that takes a lot of scrolling to find.

Fix

Truncate the standard output rendered in the summary: when it exceeds 40 lines, keep the first 30 lines (where the real error lives) and the last 10 lines (trailing diagnostics), and collapse the middle with a localized marker:

... output truncated, 660 lines omitted ...

Standard error is left untouched. Output that already fits within the budget is echoed unchanged.

Head+tail elision is used (rather than the existing live-stream tail) because it's robust across failure modes: it preserves the error at the top for the invalid-argument case and trailing output at the bottom for crashes/exceptions.

Changes

• TerminalTestReporter.cs — new TruncateOutputForSummary helper, wired into AppendExecutableSummary.
• CliCommandStrings.resx — new localized TestApplicationOutputTruncated string; all 13 .xlf files regenerated via /t:UpdateXlf.
• TerminalTestReporterTests.cs — 3 tests: small output unchanged, large output head/tail/marker behavior, and end-to-end through AssemblyRunCompleted.

Verification

• dotnet build of dotnet.csproj and dotnet.Tests.csproj: 0 warnings, 0 errors.
• Truncation algorithm validated end-to-end against the test assertions (head preserved, tail preserved, middle dropped, omitted-count correct, small input unchanged).

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[Copilot]

Pull request overview

Improves the dotnet test terminal summary for Microsoft.Testing.Platform (MTP) runs by truncating excessively large standard output payloads (e.g., full test-host help dumps on invalid arguments) so the actionable error at the top remains visible.

Changes:

• Add TerminalTestReporter.TruncateOutputForSummary and apply it when rendering failing-host stdout in AppendExecutableSummary.
• Introduce a localized truncation marker string (TestApplicationOutputTruncated) and propagate it through regenerated .xlf files.
• Add unit tests covering small-output passthrough, large-output head/tail truncation, and an end-to-end summary assertion via AssemblyRunCompleted.

Show a summary per file

File	Description
test/dotnet.Tests/CommandTests/Test/TerminalTestReporterTests.cs	Adds coverage for truncation behavior and summary rendering.
src/Cli/dotnet/Commands/Test/MTP/Terminal/TerminalTestReporter.cs	Implements and wires stdout truncation into the failure summary output.
src/Cli/dotnet/Commands/CliCommandStrings.resx	Adds the new localized truncation marker resource string.
src/Cli/dotnet/Commands/xlf/CliCommandStrings.cs.xlf	Regenerated localization entry for TestApplicationOutputTruncated.
src/Cli/dotnet/Commands/xlf/CliCommandStrings.de.xlf	Regenerated localization entry for TestApplicationOutputTruncated.
src/Cli/dotnet/Commands/xlf/CliCommandStrings.es.xlf	Regenerated localization entry for TestApplicationOutputTruncated.
src/Cli/dotnet/Commands/xlf/CliCommandStrings.fr.xlf	Regenerated localization entry for TestApplicationOutputTruncated.
src/Cli/dotnet/Commands/xlf/CliCommandStrings.it.xlf	Regenerated localization entry for TestApplicationOutputTruncated.
src/Cli/dotnet/Commands/xlf/CliCommandStrings.ja.xlf	Regenerated localization entry for TestApplicationOutputTruncated.
src/Cli/dotnet/Commands/xlf/CliCommandStrings.ko.xlf	Regenerated localization entry for TestApplicationOutputTruncated.
src/Cli/dotnet/Commands/xlf/CliCommandStrings.pl.xlf	Regenerated localization entry for TestApplicationOutputTruncated.
src/Cli/dotnet/Commands/xlf/CliCommandStrings.pt-BR.xlf	Regenerated localization entry for TestApplicationOutputTruncated.
src/Cli/dotnet/Commands/xlf/CliCommandStrings.ru.xlf	Regenerated localization entry for TestApplicationOutputTruncated.
src/Cli/dotnet/Commands/xlf/CliCommandStrings.tr.xlf	Regenerated localization entry for TestApplicationOutputTruncated.
src/Cli/dotnet/Commands/xlf/CliCommandStrings.zh-Hans.xlf	Regenerated localization entry for TestApplicationOutputTruncated.
src/Cli/dotnet/Commands/xlf/CliCommandStrings.zh-Hant.xlf	Regenerated localization entry for TestApplicationOutputTruncated.

Copilot's findings

• Files reviewed: 16/16 changed files
• Comments generated: 0