Show currently running tests in dotnet test MTP output (#49712)

[Evangelink]

Fixes #49712.

What

Surfaces currently-running tests in the dotnet test output for projects using Microsoft.Testing.Platform (MTP). The SDK already has all of the rendering machinery for this (TestProgressState/TestNodeResultsState/AnsiTerminalTestProgressFrame — including the ... and X more trimming), but it was dead code because:

1. The IPC protocol carried no test started event for MTP, only TestResultMessages.
2. TerminalTestReporterOptions.ShowActiveTests was never set.

This PR addresses both.

Changes

• New IPC message — TestInProgressMessages model + TLV serializer registered with id 10. Mirrors DiscoveredTestMessages.
• Dispatch + handler — TestApplication.OnRequest dispatches the new message; TestApplicationHandler.OnTestInProgressReceived validates the execution id and forwards each item to the reporter.
• Reporter — TerminalTestReporter.TestInProgress now takes the same (assembly, tfm, arch, executionId, instanceId, uid, displayName) shape as TestCompleted.
• State — TestNodeResultsState keys running tests by (instanceId, uid) instead of just uid, and tracks a completed set so a late in-progress notification can never resurrect a finished test. Retries (which get a new instanceId) are surfaced correctly.
• Default-on — ShowActiveTests is enabled when AnsiMode == AnsiIfPossible and progress isn't suppressed (so it stays off in CI, --no-ansi, --no-progress, and LLM environments).

Back-compat

The named-pipe protocol is not versioned for this change because TLV with unknown-id skip is sufficient:

• Old SDK + new MTP — NamedPipeServer is constructed with skipUnknownMessages: true and TestApplication.OnRequest already returns VoidResponse for UnknownMessage, so the events are dropped silently.
• New SDK + old MTP — old MTPs simply never send the message, so the running-tests panel stays empty (which is what users see today).

No protocol version bump is needed.

Producer-side gating (concern from @nohwnd)

@nohwnd flagged in #49712 that 12 parallel processes flooding test started events for fast tests would be noisy. Mitigations in this PR:

• The existing per-assembly ... and X more trimming in TestNodeResultsState.GetRunningTasks already handles bursty cases.
• The stale-add guard means stragglers that arrive after a test result don't pollute the panel.

The companion MTP-side PR is microsoft/testfx#8652, which actually emits the new TestInProgressMessages from DotnetTestDataConsumer on InProgressTestNodeStateProperty. A further producer-side slow-test threshold (e.g. only emit after N ms) could be added there as a follow-up.

Tests

• TestInProgressMessagesSerializerTests — round-trip with populated and empty payloads; pins serializer id to 10.
• TestNodeResultsStateTests — stale-add suppression after completion, retry path (new instanceId re-adds), distinct (instanceId, uid) keying.

All 6 new tests pass.

[Copilot]

Pull request overview

This PR enables live display of currently running tests in dotnet test for Microsoft.Testing.Platform by adding an IPC message for in-progress tests and wiring it into the existing terminal progress renderer.

Changes:

• Adds TestInProgressMessages IPC model, serializer, field IDs, and serializer registration.
• Dispatches in-progress test events through TestApplicationHandler into TerminalTestReporter.
• Updates active-test state tracking to key by (instanceId, uid) and adds tests for serializer and state behavior.

Show a summary per file

File	Description
src/Cli/dotnet/Commands/Test/MTP/IPC/Models/TestInProgressMessages.cs	Adds IPC records for running-test notifications.
src/Cli/dotnet/Commands/Test/MTP/IPC/ObjectFieldIds.cs	Reserves field and serializer IDs for in-progress test messages.
src/Cli/dotnet/Commands/Test/MTP/IPC/Serializers/RegisterSerializers.cs	Registers the new serializer.
src/Cli/dotnet/Commands/Test/MTP/IPC/Serializers/TestInProgressMessagesSerializer.cs	Implements TLV serialization for in-progress test messages.
src/Cli/dotnet/Commands/Test/MTP/TestApplication.cs	Routes the new message type to the handler.
src/Cli/dotnet/Commands/Test/MTP/TestApplicationHandler.cs	Validates and forwards running-test notifications to the reporter.
src/Cli/dotnet/Commands/Test/MTP/Terminal/TerminalTestReporter.cs	Adds instance-aware active-test updates and removal.
src/Cli/dotnet/Commands/Test/MTP/Terminal/TestNodeResultsState.cs	Tracks running tests by instance and suppresses stale completed entries.
src/Cli/dotnet/Commands/Test/MTP/MicrosoftTestingPlatformTestCommand.cs	Enables active-test display for ANSI progress mode.
test/dotnet.Tests/CommandTests/Test/TestInProgressMessagesSerializerTests.cs	Adds serializer round-trip and ID coverage.
test/dotnet.Tests/CommandTests/Test/TestNodeResultsStateTests.cs	Adds coverage for stale-add suppression and retry keying.

Copilot's findings

• Files reviewed: 11/11 changed files
• Comments generated: 1