build: move Copilot integration tests to platform test jobs

Move the 3 VS Code-dependent Copilot test steps (extension tests,
completions core tests, sanity tests) from the standalone Copilot job
into the Windows, Linux, and macOS platform test jobs.

- Parameterize copilot/test-steps.yml with runIntegrationTests flag
- Create copilot/test-integration-steps.yml for platform-specific setup
- Add Copilot integration tests after core tests in each platform
- product-copilot-release.yml is unaffected (uses default=true)