Drop MSBuild extension's dependency on internal TestRequestExecutionTimeInfo

[Evangelink]

What

The MSBuild extension (Microsoft.Testing.Extensions.MSBuild) no longer
depends on the internal TestRequestExecutionTimeInfo message from
Microsoft.Testing.Platform. MSBuildConsumer now measures session
duration with a local Stopwatch started in OnTestSessionStartingAsync
and stopped in OnTestSessionFinishingAsync (where the
RunSummaryInfoRequest is now sent).

Because that internal message had no other consumer (DotnetTestDataConsumer
listed it in DataTypesConsumed but never actually handled it in its
ConsumeAsync switch), this PR also:

• removes TestRequestExecutionTimeInfo.cs entirely;
• drops the IDataProducer implementation (DataTypesProduced + the
  messageBus.PublishAsync call) from TestHostTestFrameworkInvoker;
• removes the dead typeof(TestRequestExecutionTimeInfo) entry from
  DotnetTestDataConsumer.DataTypesConsumed.

Why

Tracked by #8437 ("MSBuild extension still depends on internal
TestRequestExecutionTimeInfo"), which itself was a blocker noted in
#7739 for letting the MSBuild extension drop IVT on
Microsoft.Testing.Platform.

Behavior change

The duration reported in the MSBuild summary line is now measured from the
start of MSBuildConsumer.OnTestSessionStartingAsync to the end of its
OnTestSessionFinishingAsync, instead of just the
CreateTestSessionAsync → CloseTestSessionAsync window previously
measured by TestHostTestFrameworkInvoker. In practice this difference is
typically negligible and arguably more representative of what users perceive
as the test run duration.

Test

• MSTest.Acceptance.IntegrationTests and
  Microsoft.Testing.Platform.Acceptance.IntegrationTests -- all targeted
  InvokeTestingPlatform_Target_Should* cases pass (12/12 single-TFM,
  32/32 with multi-TFM).
• New regression assertion in MSBuildTests.Test.cs checks the summary
  line still contains a non-empty Duration: ... value.
• Full unit-test suite passes locally.

Fixes #8437

[Copilot]

Pull request overview

This PR removes the MSBuild extension’s dependency on the internal TestRequestExecutionTimeInfo message by switching MSBuildConsumer to measure session duration locally (Stopwatch) and then cleaning up the now-dead message type and its unused producer/consumer wiring inside Microsoft.Testing.Platform.

Changes:

• Replaces TestRequestExecutionTimeInfo consumption in MSBuildConsumer with a session-scoped Stopwatch and sends the run summary at OnTestSessionFinishingAsync.
• Removes the internal message type (TestRequestExecutionTimeInfo.cs) and stops producing it from TestHostTestFrameworkInvoker.
• Cleans up DotnetTestDataConsumer’s DataTypesConsumed and adds an acceptance regression assertion ensuring a non-empty Duration: is still reported.

Show a summary per file

File	Description
test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/MSBuildTests.Test.cs	Adds regression assertion that MSBuild summary output still includes a non-empty Duration.
src/Platform/Microsoft.Testing.Platform/ServerMode/DotnetTest/IPC/DotnetTestDataConsumer.cs	Removes dead consumed data type entry for the deleted timing message.
src/Platform/Microsoft.Testing.Platform/Requests/TestHostTestFrameworkInvoker.cs	Stops producing/publishing the internal timing message and drops IDataProducer implementation.
src/Platform/Microsoft.Testing.Platform/Messages/TestRequestExecutionTimeInfo.cs	Deletes the internal timing message type that no longer has consumers.
src/Platform/Microsoft.Testing.Extensions.MSBuild/MSBuildConsumer.cs	Implements local session timing (Stopwatch) and emits run summary during session finishing.

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 1