Fix/global agent bug

[paullizer]

• Global Agent Scope Gate Fallback
  • Fixed per-user Semantic Kernel chats so selecting a global agent no longer silently falls back to the standard GPT model when personal agents are disabled for the tenant.
  • The per-user loader now treats global, personal, and group agent scopes separately, allowing valid global-agent selections to continue through agent invocation while keeping personal and group scope toggles enforced as configured.
  • Added regression coverage for the shared scope gate used by the per-user loader.
  • (Ref: semantic_kernel_loader.py, functions_agent_scope.py, test_global_agent_scope_gate.py, global agent request routing)

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