Emit GCSToGCSOperator deprecation warnings after rendering - #70449

Thanks — correct fix, and the best-tested version of this pattern I've seen in the current batch. source_object, source_objects and delimiter are all template fields, so emitting these deprecation warnings from __init__ checked the raw "{{ ... }}" strings: a templated wildcard never warned, and a literal one warned at parse time for every Dag processor loop rather than once per task run.

test_wildcard_deprecation_warning_uses_rendered_source_object is the part that matters — it builds a real DAG and asserts the warning fires on the rendered value. That's the actual contract, and it's what distinguishes this from a mechanical move.

Pulling the three warnings into _warn_on_deprecated_template_fields() rather than inlining them in execute also keeps execute readable.

Note there's a near-identical PR in #70621, opened three days after this one. I've suggested there that it be closed in favour of this one — this came first and has the stronger test. No action needed from you.

Two small things inline.

Drafted-by: Claude Code (Opus 5); reviewed by @potiuk before posting