Fix race condition in setSelectedPair

[kevmo314]

Description

Fixes a small race condition in setSelectedPair that results in an occasional test failure.

=== RUN   TestRoleConflict
=== RUN   TestRoleConflict/Controlling
=== RUN   TestRoleConflict/Controlled
    agent_test.go:2029: 
        	Error Trace:	/home/runner/work/ice/ice/agent_test.go:2029
        	            				/opt/hostedtoolcache/go/1.25.3/x64/src/runtime/asm_amd64.s:1693
        	Error:      	Received unexpected error:
        	            	the agent is closed
        	Test:       	TestRoleConflict/Controlled
--- FAIL: TestRoleConflict (0.81s)
    --- PASS: TestRoleConflict/Controlling (0.20s)
    --- FAIL: TestRoleConflict/Controlled (0.20s)

Reference issue

Fixes #...

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.56%. Comparing base (163dca5) to head (f54523f).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #826      +/-   ##
==========================================
+ Coverage   87.45%   87.56%   +0.11%     
==========================================
  Files          43       43              
  Lines        4537     4537              
==========================================
+ Hits         3968     3973       +5     
+ Misses        398      393       -5     
  Partials      171      171              

Flag	Coverage Δ
go	87.56% <100.00%> (+0.11%)	⬆️

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