Fix ConnManager exit code race

[David Negstad (danegsta)]

ConnManager could drop a recorded non-zero process exit code when process exit raced serve cancellation, causing HMP clients to receive the default exit code 0.

This updates the exit-code feeder to prefer an already-available ExitHandler result even when the serve context is also canceled. The helper is split out so the timing-sensitive path has deterministic regression coverage.

Validation:

• make test-prereqs
• go test -count 1 -parallel 32 ./internal/termpty
• make test
• make lint

Fixes: #199

[Copilot]

Pull request overview

This PR fixes a timing-sensitive race in ConnManager where a recorded non-zero process exit code could be dropped when process exit raced serve-context cancellation, causing HMP clients to receive the default exit code 0. Previously, the exit-code feeder goroutine used a single select between ExitHandler.Exited() and serveCtx.Done(); when cancellation won the race, it closed exitCodeCh without ever sending the code. The refactor extracts the feeder logic into a testable helper that prefers an already-available exit result even when the serve context is also canceled.

Changes:

• Extracted the exit-code feeding logic into sendProcessExitCode/sendExitCode helpers, adding an inner select that re-checks Exited() when serveCtx.Done() fires so an already-recorded exit code is still forwarded.
• Added process package import to conn_manager.go.
• Added a deterministic regression test (TestSendProcessExitCodePrefersExitedProcessAfterServeCancel) that exercises the both-ready timing scenario.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
internal/termpty/conn_manager.go	Splits the exit-code feeder into sendProcessExitCode/sendExitCode; the cancellation branch now re-checks Exited() so a recorded exit code is forwarded instead of dropped.
internal/termpty/conn_manager_test.go	Adds a regression test asserting the recorded exit code is sent when process exit and serve cancellation are both ready.

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[David Negstad (danegsta)]

/backport to release/0.25

[github-actions]

Started backporting to release/0.25: https://github.com/microsoft/dcp/actions/runs/28544273580

[github-actions]

David Negstad (@danegsta) backport PR couldn't be created automatically, please create the backport PR manually!

Open backport PR into release/0.25.