Allure integration with xUnit.net v3 MTP

[delatrie]

Note

This PR will be merged as soon as the required changes are introduced in Allure.Net.Commons to support steps, fixtures, and the runtime API - this is a work in progress now.

Summary

Adds the new Allure.Xunit.v3 adapter for xUnit.net v3 projects running on Microsoft Testing Platform.

The integration:

• registers Allure through the MTP builder hook when UseMicrosoftTestingPlatformRunner=true;
• reports xUnit.net v3 lifecycle events, metadata, parameters, and failures to Allure; does not support fixtures, steps, and attachments yet (Runtime API support will be added soon).
• ships a source generator that applies the assembly-level setup and generates Allure-aware entry point helpers;
• supports Allure test plans with early xUnit.net pre-execution filtering;
• allows custom Allure registration and custom entry points through AddAllureXunit, AllureXunitEntryPoint, and AllureXunitRunner.

This also updates the shared MTP/testing infrastructure with cancellation support, sample-project MSBuild property support, stronger assertions, and global error/attachment checks.

Documentation

Adds README coverage for:

• required packages and UseMicrosoftTestingPlatformRunner;
• out-of-box result location;
• basic attribute usage;
• configuration discovery;
• test plan behavior;
• custom Allure registration;
• custom entry points;
• supported MSBuild properties and troubleshooting.

Public XML documentation was added and polished for the new xUnit v3 APIs and generator-produced helpers.

Extra changes

• Use the Publish configuration for packaging in test.yml.
• Add support for solution-wide sample properties to integration test projects.
• Add support for MTP TestResults as the default parent directory for allure-results to the sample runner.
• Add support for test result cancellation (discarding) to Allure.TestingPlatform as the last resort for test plan filtering.
• Add global attachment/error assertions to Allure.Testing.
• Improved failed assertion messages.
• Switch from strict ordered matching to perfect matching (all items in any order) for root collections (test results, containers, globals).
• Ignore pre-run mode for tests that depends on custom configuration, arguments, or environment.
• Provide the ReadAllureResults API from the sample runner to test projects.
• Do not throw an exception if no Allure results were produced in the pre-run flow. This should be up to the test to decide is it correct or not.