Fix template-field validation timing in PapermillOperator

[bramhanandlingala]

Fixes the papermill provider's entry from the #70296 exemption-list burn-down: PapermillOperator.

input_nb and output_nb are template fields, but were validated in __init__ — before Jinja templating runs, so the check ran against the raw un-rendered value.

• Moved validation from __init__ to execute(), after templating occurs.
• __init__ now only does plain assignments.
• Removed PapermillOperator from the exemption list.
• Updated existing test + added a new test covering the fixed behavior.

Related to #70296

[shahar1]

See #70437 (review)