Fix OTel review comments: typo, IDisposable, deduplicate completed co…

[Evangelink]

…unter, add tests

• Fix typo: _totalUnknownedTests -> _totalUnknownTests
• Implement IDisposable on OpenTelemetryResultHandler to clean up orphaned activities
• Make TestApplicationResult implement IDisposable to forward disposal
• Move _totalCompletedTests.Add(1) into HandleTestResult to deduplicate
• Add 19 unit tests for OpenTelemetryResultHandler covering all Notify methods, activity lifecycle, tag setting, disposal, and edge cases

[Copilot]

Pull request overview

Updates Microsoft.Testing.Platform’s OpenTelemetry result handling to address review feedback by fixing a typo, ensuring OTel activities are cleaned up via IDisposable, and improving counter accounting; adds unit tests to cover the handler’s notify paths and activity lifecycle.

Changes:

• Rename _totalUnknownedTests to _totalUnknownTests.
• Implement IDisposable on OpenTelemetryResultHandler and forward disposal from TestApplicationResult to clean up orphaned activities.
• Deduplicate completed-test counting by moving _totalCompletedTests.Add(1) into HandleTestResult, and add comprehensive unit tests for the handler.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
test/UnitTests/Microsoft.Testing.Platform.UnitTests/Telemetry/OpenTelemetryResultHandlerTests.cs	Adds unit tests covering notify methods, activity tagging/disposal, and edge cases.
src/Platform/Microsoft.Testing.Platform/Telemetry/OpenTelemetryResultHandler.cs	Fixes counter field typo, adds IDisposable to dispose orphaned activities, and deduplicates completed counter updates.
src/Platform/Microsoft.Testing.Platform/Services/TestApplicationResult.cs	Implements IDisposable to dispose the OTel result handler during shutdown.