Add options_mapping support to HITLBranchOperator

[cmarteepants]

This PR enhances HITLBranchOperator to accept an optional options_mapping dict that maps human-facing option labels to downstream task_ids. Previously, HITL branching required users to select raw task_ids, which isn’t user-friendly in business-facing workflows. With this change, a workflow can present clearer options (e.g., “Full Deploy” / “Canary”) while still resolving to valid downstream tasks.

Changes

• New parameter: options_mapping: dict[str, str] | None
  • Keys: option labels (same as what's provided to options)
  • Values: task_ids of valid downstream tasks
• Validation:
  • Ensures keys are valid options
  • Ensures values are strings (task_ids)
• Execution:
  • Normalizes chosen options and applies mapping before branching
  • Falls back to the raw option if no mapping is provided
• Tests:
  • Single and multiple mapped choices
  • Fallback to unmapped options
  • Error when mapping points to non-downstream tasks (AirflowException)
  • Error on invalid keys
  • Error on non-string mapping values

Rationale
This improves HITL usability for end-users by decoupling the labels people choose from the task_ids Airflow needs that may not necessarily be known by the approver, while keeping strong validation in place.

Example

op = HITLBranchOperator(
    task_id="choose",
    subject="Approval required",
    options=["Approve", "Reject"],
    options_mapping={"Approve": "publish", "Reject": "archive"},
)
op >> [EmptyOperator(task_id="publish"), EmptyOperator(task_id="archive")]

If the user selects Approve, the operator branches to the publish task.