fix(conn): plug goroutine leaks and WaitGroup race in ssh/agent packages

[skevetter]

Summary

Re-applies the 3 bug fixes from PR #90 (reverted in PR #111):

• helper.go goroutine leak: setupContextCancellation returned error but spawned a goroutine selecting on its own exit channel, making it impossible to unblock. Changed to return (func(), error) with a done channel; caller defers cleanup.
• forward.go WaitGroup race: In forward() and reverseForward(), the first goroutine used go func() + defer wg.Done() without a preceding wg.Add(1), racing with wg.Wait(). Converted both to waitGroup.Go().
• inject.go pipe orphan: pipe() spawned 2 io.Copy goroutines but returned on the first error, orphaning the second forever. Changed signature to accept io.WriteCloser/io.ReadCloser and close both endpoints after the first copy finishes.

Adds comprehensive test suites for pipe(), readLine(), waitForMessage(), and performMutualHandshake().

[netlify]

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⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 3afdc8c8-25a8-4e0c-91c5-7fac905e9c73

📥 Commits

Reviewing files that changed from the base of the PR and between 42418d1 and 9858cb2.

📒 Files selected for processing (4)

• pkg/inject/inject.go
• pkg/inject/inject_test.go
• pkg/ssh/forward.go
• pkg/ssh/helper.go

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