Add test to catch missing extension YAML files in DefaultExtensionCatalog

[benbellick]

While adding functions_aggregate_decimal_output to DefaultExtensionCatalog, we noticed there was no way to catch this kind of omission. Added a test that dynamically discovers all extension YAMLs on the classpath and verifies they're loaded, so we don't silently miss new extensions again.

Also added the other missing files found by the test: functions_set and functions_geometry. functions_list remains excluded due to #688.