Simplify airflowctl query parameter handling

[henry3260]

Why

airflowctl API operations need to build query parameters consistently when optional filters are added. The Dag run list operation already had several repeated checks for skipping unset values and serializing datetimes, and similar optional parameter handling exists in other operations. Centralizing that logic makes future filters easier to add without duplicating the same query parameter handling.

suggested by @bugraoz93 in #68564 (comment)

What

• Added helper functions in airflow-ctl/src/airflowctl/api/operations.py to build query parameters and serialize datetime values with isoformat().
• Updated DagRunOperations.list() to use the shared helper for state, date, logical date, ordering, and limit query parameters.
• Updated JobsOperations.list() to use the same helper while preserving the existing behavior for empty job_type and hostname values.
• Added a unit test in airflow-ctl/tests/airflow_ctl/api/test_operations.py covering skipped None values and datetime serialization.

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[henry3260]

For now, I prefer using a helper function instead of a decorator because there are only two call sites. We can revisit a decorator-based approach later if more airflowctl operations need the same query parameter handling.