fix(contextutil): prevent goroutine leak on context cancellation

[G360-Niek]

What

ExecFunc, ExecFuncWithTwoReturns, and ExecFuncWithThreeReturns run fn in a goroutine and return via ctx.Done() when the context is cancelled before fn finishes. The worker then sends its result on an unbuffered channel that no longer has a receiver and blocks forever — leaking one goroutine (and, through reader.ConnReadN, the underlying connection) per cancelled call.

This buffers each result channel (cap 1) so the worker can always send and exit even after the caller has returned.

Proof (added regression tests)

context/leak_test.go drives 50 context-cancelled calls per function whose fn outlives the context, then asserts the goroutine count returns to baseline.

• Before (unbuffered): all three fail — ~50 goroutines still alive after 50 context-cancelled calls.
• After (buffered): all three pass in ~0.3s; the full context/ suite and -race stay green.

Scope

This fixes the root leak in the ExecFunc* helpers, which also makes reader.ConnReadN fully cleanable — a caller that closes the connection on cancellation now leaks nothing. The one residual case (a ConnReadN whose caller never closes the connection leaves the worker parked in FD.Read) would need ConnReadN to close / set a read deadline on cancel; happy to follow up separately if you'd like that addressed here too.

Refs #759