fix(tests): validate provider types exist in backward compat test

The backward compatibility test previously only validated YAML schema
structure but did not verify that referenced provider types actually
exist in the registry. This would have allowed breaking changes like
renaming "meta-reference" to "builtin" (236a230) to pass without
detection(although the PR was correctly marked as a breaking change).

This commit enhances the test to use `validate_and_prepare_providers()`,
the same validation function the stack uses at runtime, which now checks:
- Provider types exist in registry
- No conflicts with auto-routed APIs
- Provider deprecation warnings/errors

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
Signed-off-by: Derek Higgins <derekh@redhat.com>

Author: derekhiggins

tests/backward_compat/test_run_config.py

@@ -18,6 +18,8 @@
 import yaml
 
 from llama_stack.core.datatypes import StackConfig
+from llama_stack.core.distribution import builtin_automatically_routed_apis, get_provider_registry
+from llama_stack.core.resolver import validate_and_prepare_providers
 from llama_stack.core.stack import replace_env_vars
 
 
@@ -58,4 +60,19 @@ def test_load_run_config(config_file):
     except Exception:
         pass
 
-    StackConfig.model_validate(config_data)
+    # Validate schema
+    config = StackConfig.model_validate(config_data)
+
+    # Validate providers using the same validation the stack uses at runtime
+    # This validates:
+    #   - Provider types exist in registry
+    #   - No conflicts with auto-routed APIs
+    #   - Provider deprecation warnings/errors
+    provider_registry = get_provider_registry()
+    routing_table_apis = {x.routing_table_api for x in builtin_automatically_routed_apis()}
+    router_apis = {x.router_api for x in builtin_automatically_routed_apis()}
+
+    try:
+        validate_and_prepare_providers(config, provider_registry, routing_table_apis, router_apis)
+    except ValueError as e:
+        pytest.fail(f"Provider validation failed: {str(e)}")