Consolidate device-to-queue mapping into constants.py

[khluu]

Summary

• Add a DEVICE_TO_QUEUE dict in constants.py that serves as the single source of truth for mapping DeviceType to AgentQueue (covers all 26 device types with straightforward 1:1 mappings)
• Replace the 45-line if-elif chain in get_agent_queue() with a concise dict lookup against DEVICE_TO_QUEUE, keeping only the branch-dependent docker logic and GPU-count fallback as special cases
• Remove the duplicate 12-entry AMD queue mapping dict from _create_amd_mirror_step(), replacing it with a DEVICE_TO_QUEUE.get() call

Test plan

• Verify pipeline generation produces identical output for existing step configurations (all device types should resolve to the same queues as before)
• Verify docker image build steps still use branch-dependent queues (premerge vs postmerge)
• Verify AMD mirror steps still resolve to correct AMD queues
• Verify steps with num_devices 2 or 4 (and no explicit device type) still fall back to GPU_4
• Verify default fallback to GPU_1 still works for unrecognized devices

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