fix: keep existing workout step payloads unchanged, add target tests

[cyberjunky]

Follow-up to #440.

• No change for existing steps: create_interval_step / create_distance_interval_step without targets upload exactly as before Add support for targeted workouts #440 again. The interval step and no.target displayOrder values are restored, and a no.target secondaryTargetType is no longer added to every step.

• Cleanup: the two Protocol classes and per-class field redeclarations are replaced with shared _RangeTarget / _ZoneTarget base classes; IntensityTarget is now their union. The targeted helpers call the step functions directly, without type: ignore.

• Docs: PaceTarget notes that limits are in m/s, so the slower pace is the lower limit.

• Tests: new tests/test_workout_targets.py covers every target type, zone vs range targets, secondary targets, distance steps, limit validation, pace_to_mps / speed_to_mps, full workout serialization, and pins the untargeted step payload.

• Demo: the running sample workout now has a pace target on its intervals (PaceTarget + pace_to_mps), and the cycling sample a 220-250 W power target with an 85-95 rpm cadence secondary target, so the demo upload options exercise the new helpers.

The payload for targeted steps is unchanged from #440, which was tested live by the contributor.

Summary by CodeRabbit

• New Features
  • Workout interval targets now support validated range and zone targets, including power, heart-rate, speed, and pace targets.
  • Targeted time- and distance-based intervals accept these targets, including optional secondary targets.
• Bug Fixes
  • Corrected interval step display ordering and preserved the distinction between an absent secondary target and an explicit no-target value.
  • Pace target documentation now clarifies that the lower limit represents a slower pace.

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📒 Files selected for processing (4)

• garminconnect/workout.py
• test_data/sample_cycling_workout.py
• test_data/sample_running_workout.py
• tests/test_workout_targets.py

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