Add global.json

[mbg]

Risk assessment

For internal use only. Please select the risk level of this change:

• Low risk: Changes are fully under feature flags, or have been fully tested and validated in pre-production environments and are highly observable, or are documentation or test only.
• High risk: Changes are not fully under feature flags, have limited visibility and/or cannot be tested outside of production.

Which use cases does this change impact?

• Advanced setup - Impacts users who have custom workflows.
• Default setup - Impacts users who use default setup.
• Code Scanning - Impacts Code Scanning (i.e. analysis-kinds: code-scanning).
• Code Quality - Impacts Code Quality (i.e. analysis-kinds: code-quality).
• Third-party analyses - Impacts third-party analyses (i.e. upload-sarif).
• GHES - Impacts GitHub Enterprise Server.

How did/will you validate this change?

• Test repository - This change will be tested on a test repository before merging.
• Unit tests - I am depending on unit test coverage (i.e. tests in .test.ts files).
• End-to-end tests - I am depending on PR checks (i.e. tests in pr-checks).
• Other - Please provide details.
• None - I am not validating these changes.

If something goes wrong after this change is released, what are the mitigation and rollback strategies?

• Feature flags - All new or changed code paths can be fully disabled with corresponding feature flags.
• Rollback - Change can only be disabled by rolling back the release or releasing a new version with a fix.
• Other - Please provide details.

How will you know if something goes wrong after this change is released?

• Telemetry - I rely on existing telemetry or have made changes to the telemetry.
  • Dashboards - I will watch relevant dashboards for issues after the release. Consider whether this requires this change to be released at a particular time rather than as part of a regular release.
  • Alerts - New or existing monitors will trip if something goes wrong with this change.
• Other - Please provide details.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Consider adding a changelog entry for this change.
• Confirm the readme and docs have been updated if necessary.

[Copilot]

Pull Request Overview

This PR adds a global.json file to the multi-language test repository to specify the .NET SDK version for building the C# test code.

• Adds .NET SDK version pinning (9.0.307) with forward-rolling behavior to the test repository
• Configures the SDK to use "latestFeature" roll-forward policy for flexibility in CI environments