Add lightweight routing-only test to validate skill selection across models on every PR

### Problem

Skill selection is only fully tested through slow, comprehensive test suites, limiting our ability to catch regressions early on every PR.

### Desired Solution

Add a light, fast test that validates routing-only/skill selection for all models, omitting full content quality checks. This can run on every PR without slowing down the workflow.

### Alternatives and Workarounds

Continue only using full test suites, or try to speed up those suites but risk missing skill selection bugs on PRs.

### Impact

Faster detection of routing/skill selection errors; prevent regressions from being merged due to lack of early feedback; allows full test suites to focus on deep correctness.

### Additional Context

Request originated in [Issue #1647](https://github.com/microsoft/GitHub-Copilot-for-Azure/issues/1647). Intended to complement, not replace, full suite coverage.

Locate the existing full skill-selection test suites and the pull-request workflow that runs them. Start by identifying how models are enumerated and how routing-only behavior can be isolated; done means a fast test covers all models on every PR without running full content-quality checks.

Written by the indexing model from the issue text.