Apply dotnet test zero-tests verdict at the whole-run level (microsoft/testfx#7457)

[Evangelink]

Summary

Implements the dotnet test (Microsoft.Testing.Platform path) side of microsoft/testfx#7457: make the "zero tests ran" (exit code 8) verdict apply to the whole run instead of failing once per test module.

Today TestApplicationActionQueue aggregates the raw per-module exit code. When a run uses --test-modules or a global --filter, a module that matches no tests exits with 8 (ZeroTests), which poisons the aggregate exit code even though other modules ran fine. There is no way to get a single run-level "zero tests" result short of --ignore-exit-code 8 (which suppresses it everywhere).

Change

• TestApplicationActionQueue: normalize a per-module ExitCode.ZeroTests (8) to Success before aggregating. An individual module that matched no tests is no longer treated as a whole-run failure.
• MicrosoftTestingPlatformTestCommand.Run: decide the whole-run zero-tests verdict once, from output.TotalTests. When no global --minimum-expected-tests is set and the run executed zero tests in total, the command returns ExitCode.ZeroTests (8). When a global --minimum-expected-tests N is set, the existing aggregate check against output.TotalTests governs.

Behavior

• Single project / all modules empty (e.g. a typo in --filter): total tests 0 → still exit code 8 (unchanged).
• Some modules have tests, others match nothing (--test-modules / global filter): run succeeds instead of failing with 8 — this is the requested fix.
• dotnet test --minimum-expected-tests N: global check on the aggregate total (unchanged).
• dotnet test -- --minimum-expected-tests N (forwarded per-module): a module that runs fewer than N tests fails with ExitCode.MinimumExpectedTestsPolicyViolation (9), which is not normalized here and is preserved. Both a global and a forwarded minimum can be combined and are honored independently.

Why normalize 8 rather than flow --minimum-expected-tests 0 down

Flowing --minimum-expected-tests 0 to each module was considered but rejected: older Microsoft.Testing.Platform versions reject --minimum-expected-tests 0 (it required a positive value), and the SDK does not know a module's MTP version at argument-build time, so flowing it would break those modules with an invalid-command-line error. Normalizing the child 8 is an SDK-only interpretation with no MTP-version dependency.

Depends on / pairs with

microsoft/testfx PR #9709: it makes an explicit --minimum-expected-tests N violation return exit code 9 (not 8) even when zero tests ran. That is what lets this PR safely normalize 8 without swallowing a forwarded per-module minimum. With an older MTP that still returns 8 for a forwarded minimum + zero tests, that narrow edge would be normalized to success; with the paired testfx change it is reported correctly as 9.

Notes

• ⚠️ Draft: this was authored without a local dotnet/sdk build. Needs CI to compile and run, and SDK-side tests should be added (e.g. a multi-module scenario where one module matches no tests asserts a successful run, and an all-empty run asserts exit code 8).

Fixes the dotnet test side of microsoft/testfx#7457.