chore(deps): update test and workflow dependencies

[Codewriter90x]

Summary

Consolidates the remaining Dependabot updates after the initial public repository bootstrap.

Changes

• updates GitHub-maintained Actions to their current Node.js 24-compatible major versions;
• updates Microsoft.NET.Test.Sdk to 18.9.0;
• updates xunit.runner.visualstudio to 4.0.0;
• retains xUnit 2.9.3 merged separately in Bump xunit from 2.9.2 to 2.9.3 #7.

This supersedes Dependabot PRs #1–#6 and #8. No gameplay, Unity runtime, website content, licensing, or release artifacts are changed.

Validation

• public readiness contracts: passed;
• asset provenance contracts: passed;
• macOS signing contracts: passed;
• Windows packaging contracts: passed;
• build: 0 warnings, 0 errors;
• Core tests: 42/42;
• MIDI capture tests: 39/39.