feat: add uipath-testing skill for Test Suite automation

[1aifanatic]

Summary

• Adds a new skills/uipath-testing/ skill covering the full UiPath Test Suite lifecycle
• Covers coded test cases (C# with [TestCase] attribute), XAML test cases, and the project.json fileInfoCollection structure
• Documents all assertion patterns (testing.VerifyExpression, VerifyExpressionWithOperator) with practical examples
• Covers data-driven testing with CSV files via dataVariations in project.json
• Documents Test Manager CLI (uip tm) for test sets, schedules, executions, and report export
• Includes a GitHub Actions CI/CD integration example with non-interactive login
• Documents mocking and isolation patterns: test-specific Orchestrator folders, assets, queues, and local mock servers
• Common pitfalls table covering CI/CD failures, data-driven setup issues, and test isolation

Motivation

The existing uipath-platform skill mentions Test Manager briefly, and uipath-coded-workflows covers test case structure. However, there is no dedicated skill that guides AI agents through the complete testing workflow — from writing test cases to scheduling nightly runs and exporting reports. This skill fills that gap.

Test plan

• Verify SKILL.md frontmatter format matches existing skills
• Verify uip tm CLI command syntax matches actual CLI (uip tm projects list, uip tm testsets create, etc.)
• Verify coded test case structure matches uipath-coded-workflows skill conventions (same project.json fileInfoCollection format)
• Verify GitHub Actions example uses correct CLI auth flags