flake: ptytest matcher receives no output #1400

Various tests affected, underlying cause is pty not getting any output to match:

• CI run: https://github.com/coder/coder/actions/runs/23049972150
• Job: https://github.com/coder/coder/actions/runs/23049972150/job/66949081715 (test-go-pg ubuntu-latest)
• Commit: ff156772f2022fb27acabdd8347d6c3e4faf9c31 (Mathias Fredriksson) "fix(coderd/database): move context creation to first use in migration tests"

Failure details:

• agent/agentssh TestNewServer_CloseActiveConnections/Shutdown_Early
• agentssh_test.go:249: timeout
• ptytest.go:170: match deadline exceeded: context deadline exceeded (wanted "started"; got "")
• ptytest.go:102: copy did not close in time

No data race warnings, panic, or OOM indicators found in the job logs.

Assignment analysis:

• git blame -L 149,260 agent/agentssh/agentssh_test.go (local blame recommended)
• git log --oneline -10 --follow agent/agentssh/agentssh_test.go -> ebad5c3 "test(agent): fix channel timeout in TestNewServer_CloseActiveConnections" by Mathias Fredriksson (most recent meaningful change to this test).