⚡ Bolt: [performance improvement] Optimize dictionary lookup in _add_phase_tracking_costs

[dieterolson]

💡 What: Replaced a double dictionary lookup (if key in dict: val = dict[key]) with a single get() call (val = dict.get(key)) in the _add_phase_tracking_costs hot loop.
🎯 Why: To reduce unnecessary hash map lookups during trajectory optimization setup, adhering to Python best practices for O(1) lookups in tight loops.
📊 Impact: Micro-optimization. Reduces the overhead of setting up phase tracking costs in Drake's MathematicalProgram.
🔬 Measurement: Verified that tests and benchmarks continue to pass successfully.

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PR created automatically by Jules for task 5839190333747016691 started by @dieterolson

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[dieterolson]

Closing in favor of consolidated batch PR #311.