feat: add backward compatibility tests for run.yaml (#3952)

This adds automated backward compatibility testing for `run.yaml` files.
As we evolve `StackRunConfig`, changes can inadvertently break existing
user configurations. This workflow catches those breaks before merge.

We test old run.yaml files (from main and the latest release) against
the PR's new code. If configs that worked before now fail, the PR is
blocked unless explicitly acknowledged as a breaking change.

**Two test layers:**
- Schema validation: Quick pytest checks that configs parse without
errors
- Integration tests: Full test suite execution to catch runtime semantic
issues (cross-field validations, provider initialization, etc.)

**What we test against:**
- main branch: Breaking changes here block the PR (this is the gate)
- Latest release: Informational only - shows if we've drifted from what
users have

If tests fail, the PR author must acknowledge the breaking change by
adding `!:` to the PR title (e.g., `feat!: change xyz`) or including
`BREAKING CHANGE:` in a commit message. Once acknowledged, the check
passes with a warning.

These jobs are run:
1. `check-main-compatibility` - Schema validation of all distribution
run.yaml files from main
2. `test-integration-main` - Full integration test suite using main's
ci-tests run.yaml
3. `test-integration-release` - Integration tests with latest release
config (informational)
4. `check-schema-release-compatibility` - Schema checks against release
(informational)

The integration tests catch issues that schema validation alone would
miss, like assertion failures in
`StackRunConfig.validate_server_stores()` or provider-specific runtime
logic.

Resolves #3311
Related to #3237

Author: ashwinb

.github/workflows/README.md

@@ -4,6 +4,7 @@ Llama Stack uses GitHub Actions for Continuous Integration (CI). Below is a tabl
 
 | Name | File | Purpose |
 | ---- | ---- | ------- |
+| Backward Compatibility Check | [backward-compat.yml](backward-compat.yml) | Check backward compatibility for run.yaml configs |
 | Update Changelog | [changelog.yml](changelog.yml) | Creates PR for updating the CHANGELOG.md |
 | API Conformance Tests | [conformance.yml](conformance.yml) | Run the API Conformance test suite on the changes. |
 | Installer CI | [install-script-ci.yml](install-script-ci.yml) | Test the installation script |