Pydantic/fastAPI no longer handle generic Mapping for Config classes

This all worked before but now we have to force the Config
to a plain dict before it will be accepted.

python/lsst/daf/butler/server.py

@@ -133,13 +133,13 @@ def read_server_config() -> Mapping:
     db: <butlerRoot>
 """
     config = Config.fromString(config_str, format="yaml")
-    return config
+    return config.toDict()
 
 
 @app.get("/butler/v1/universe", response_model=dict[str, Any])
 def get_dimension_universe(butler: Butler = Depends(butler_readonly_dependency)) -> DimensionConfig:
     """Allow remote client to get dimensions definition."""
-    return butler.dimensions.dimensionConfig
+    return butler.dimensions.dimensionConfig.toDict()
 
 
 @app.get("/butler/v1/uri/{id}", response_model=str)