global-monitor: fix crash when IBRL and multicast users share a client IP

[snormore]

Summary of Changes

• Fix global monitor crash on startup when a client has both an IBRL and a Multicast user account with the same client IP but connected to different devices (e.g., frankry for IBRL vs fr2-dzx-001 for Multicast)
• The UsersByClientIP map could arbitrarily return the Multicast user, whose device code mismatches the status device code (which correctly prefers the non-multicast tunnel), causing a fatal "user device code does not match status device code" error
• Populate User.UserType from onchain data (was previously always zero-value) and prefer non-multicast users in the UsersByClientIP map, matching the existing parseStatus logic

Diff Breakdown

Category	Files	Lines (+/-)	Net
Core logic	1	+23 / -1	+22
Tests	1	+75 / -0	+75

Small targeted bugfix with thorough test coverage.

Key files (click to expand)

• telemetry/global-monitor/internal/dz/serviceability_test.go — add test for duplicate client IP scenario verifying non-multicast user is preferred
• telemetry/global-monitor/internal/dz/serviceability.go — populate UserType field, prefer non-multicast user in UsersByClientIP map

Testing Verification

• Added unit test that creates both multicast and IBRL users with the same client IP on different devices (multicast listed first) and verifies the IBRL user wins in the map
• All existing serviceability and status tests pass