Disambiguate TRX filenames when running multiple test modules

[Evangelink]

Fixes microsoft/testfx#7345.

When dotnet test orchestrates more than one test module (via --test-modules glob or a multi-project / multi-TFM solution), the same --report-trx-filename value was forwarded verbatim to every module, so each module's TRX overwrote the previous one.

Approach

The SDK now disambiguates the file name itself when more than one module is being run, leaving the single-module behavior untouched. The substitution is done by the SDK rather than by passing MTP's placeholder tokens, so the fix works against older versions of Microsoft.Testing.Platform that don't yet support the {asm}/{tfm} placeholders introduced in 2.3.0-preview.26274.1 (microsoft/testfx#8223).

Behavior

• Single module: unchanged (no rewrite).
• Multi-module + --report-trx-filename foo.trx → rewritten to foo_<asm>_<tfm>.trx per module. No timestamp is added here so a user who explicitly chose a stable file name keeps the same semantics they get today with MTP.
• Multi-module + --report-trx only → SDK injects --report-trx-filename <asm>_<tfm>_<yyyy-MM-dd_HH_mm_ss>.trx. The UTC timestamp mirrors MTP's natural default (<user>_<machine>_<time>.trx) so re-runs don't trip the Warning: Trx file '...' already exists and will be overwritten. message.
• User-supplied {asm} / {pname} / {pid} placeholder → left alone (already unique per process). {tfm} alone is not considered unique — two modules can share a TFM, which is exactly the original bug.
• Preserves directory portion of paths (subdir/foo.trx → subdir/foo_<asm>_<tfm>.trx).
• Handles both --opt value and --opt=value forms.
• Sanitizes invalid filename chars.
• For --test-modules (where TargetFramework is null), infers TFM by walking parent directories of TargetPath and parsing each segment as a TFM folder name (with a starts-with "net" pre-filter so RIDs like win-x64 aren't mistaken for frameworks).

Testing

• 15 new unit tests in TrxReportArgumentsRewriterTests (incl. theory cases → 17 executed) covering: single-module pass-through, multi-module rewrite, equals-form args, {asm}/{pname}/{pid} short-circuit, {tfm} not short-circuiting, directory preservation, TFM inference from path (including RID layouts), no-extension case, unrelated arg preservation, and the timestamp-disambiguated injected default + re-run uniqueness.
• 17 nearby existing tests (TestApplicationHandlerTests, TestModulesFilterHandlerTests, TestCommandValidationTests) still pass.
• Built clean (0 warnings / 0 errors).

[Copilot]

Pull request overview

Disambiguates the TRX file name passed to Microsoft.Testing.Platform's TRX report extension when the SDK orchestrates multiple test modules, so each module writes a distinct file instead of overwriting siblings. Substitution is performed by the SDK so the fix works against MTP versions that predate the {asm}/{tfm} placeholders.

Changes:

• New TrxReportArgumentsRewriter rewrites --report-trx[-filename] per module (preserving directory, handling = form, sanitizing, inferring TFM from TargetPath for --test-modules), with timestamp injection when only --report-trx was passed.
• Plumbs an isMultiTestModule flag from MicrosoftTestingPlatformTestCommand through TestApplicationActionQueue to TestApplication, where the rewriter is invoked when assembling the child process arg list.
• Adds TotalTestModuleCount to ITestHandler (and both MSBuildHandler / TestModulesFilterHandler) so the command knows whether it is running >1 module.

Show a summary per file

File	Description
src/Cli/dotnet/Commands/Test/MTP/TrxReportArgumentsRewriter.cs	New rewriter that produces a unique TRX filename per module.
src/Cli/dotnet/Commands/Test/MTP/TestApplication.cs	Stores _isMultiTestModule and routes test arguments through the rewriter.
src/Cli/dotnet/Commands/Test/MTP/TestApplicationActionQueue.cs	Threads isMultiTestModule through to TestApplication construction.
src/Cli/dotnet/Commands/Test/MTP/MicrosoftTestingPlatformTestCommand.cs	Computes isMultiTestModule from testHandler.TotalTestModuleCount.
src/Cli/dotnet/Commands/Test/MTP/ITestHandler.cs	Adds TotalTestModuleCount to the handler contract.
src/Cli/dotnet/Commands/Test/MTP/MSBuildHandler.cs	Implements TotalTestModuleCount by summing per-group module counts.
src/Cli/dotnet/Commands/Test/MTP/TestModulesFilterHandler.cs	Implements TotalTestModuleCount from the resolved test-module path list.
test/dotnet.Tests/CommandTests/Test/TrxReportArgumentsRewriterTests.cs	New unit tests covering pass-through, rewrite, equals form, placeholder short-circuit, directory preservation, TFM inference, no-extension, and timestamp uniqueness.

Copilot's findings

• Files reviewed: 8/8 changed files
• Comments generated: 0

[Youssef1313]

The "test module" naming here is confusing. Looking at the name I would expect this to apply only to --test-modules, but it's not.

[Youssef1313]

I'm not convinced SDK should be concerned about any of that.

Again, I think this is purely an MTP feature, the feature was provided, and users are expected to update to latest MTP.

I wouldn't complicate the implementation in SDK and introduce additional maintenance and potential risks just because "people might be on older MTP version that doesn't support placeholders".

This even violates the initial designs around MTP that we think shipping via NuGet out of SDK release cycle helps us getting fixes to the users faster.

[Evangelink]

I'll fix by doing a change in the default name directly in MTP