chore: test flake: TestAgent_ReconnectingPTY/Screen

[johnstcn]

Seen here: https://github.com/coder/coder/actions/runs/16450706445/job/46494924455?pr=18991

    pty.go:64: Terminal contents:
        Error: Cannot find master process to attach to!                                 
    pty.go:67: Bytes Read: "\r\nError: Cannot find master process to attach to!\r\n"
    agent_test.go:1851: 
        	Error Trace:	/home/runner/work/coder/coder/agent/agent_test.go:1851
        	Error:      	Received unexpected error:
        	            	EOF
        	Test:       	TestAgent_ReconnectingPTY/Screen
        	Messages:   	find prompt