YARN is the dominant use case, and the value of master for YARN is the
literal string 'yarn' (not a URL) — there is no useful variation. Making
it a default saves every YARN user from typing the same value every time:
save_connection(name='prod') # master defaults to 'yarn'
save_connection(name='dev', master='spark://10.0.0.5:7077') # override
Added a field_validator on master that catches common typos
('yarn-cluster', 'http://...', empty string) at save time with a clear
error message instead of letting them reach spark-submit.
Backward compatible: existing Connection records without an explicit
master field default to 'yarn' on load (Pydantic applies defaults on
deserialization), so already-saved data still works.
3 new tests cover the default, the validator, and that other cluster
managers (spark://, k8s://, local[N]) still pass.
Claude
516541a4df