Track supported model list in a single configuration source for automated testing

### Problem

There isn't a single source of truth for the supported model list; this makes keeping test coverage consistent across models more challenging.

### Desired Solution

Track the supported model list in a config file or directly in the workflow matrix, so that new models are included in all standard test suites automatically.

### Alternatives and Workarounds

Continue manually updating various locations when new models are added, with risk of missing coverage.

### Impact

Reduces manual effort and human error; ensures every model gets tested consistently; helps onboard new model support faster.

### Additional Context

See [Issue #1647](https://github.com/microsoft/GitHub-Copilot-for-Azure/issues/1647) for historical context.

Start by reviewing the workflow matrices and the locations where the supported model list is currently duplicated, then read Issue #1647 for historical context. Done means one configuration source or workflow matrix supplies every standard test suite, so adding a model requires only one update.

Written by the indexing model from the issue text.