executeQueryInternal called cmd.Wait() before waiting for the goroutines streaming stdout/stderr. exec.Cmd.Wait closes the parent ends of the pipes as soon as the process exits, so on context cancellation the function could return — and read stderrBuf for the failure message — while a stream goroutine was still writing into it. That concurrent read/write is the data race in #134 (reproducible with go test -race -run TestExecuteQueryInternal_ContextTimeout). The same early close could also silently truncate stdout mid-parse and drop the tail of captured stderr.

Rather than joining the stream goroutines on the cancellation path, this restructures the shutdown sequence so the situation can't arise:

• Streams are drained before the process is reaped. g.Wait() now runs before cmd.Wait(), so Wait's pipe close can never race with (or truncate) an in-flight read. Failure errors now always carry the complete captured stderr.
• The inner goroutine/select in the stream helpers is gone. streamOutput is an inline io.Copy and the errgroup calls getQueryResult directly; both report the context error when the context has ended. With no goroutine outliving the call, the race is fixed structurally.
• A watchdog guarantees the reads can't block forever. Reordering removes the liveness guarantee cmd.Wait's force-close used to provide: if a descendant of the killed bazel process inherits the pipe write ends and holds them open (go.dev/issue/23019), the reads would never see EOF. If the command context ends and the streams haven't finished within execcmd.GracePeriod + 5s — after the SIGTERM→SIGKILL sequence has had its chance to produce a natural EOF — the watchdog force-closes the parent pipe ends to unblock them.

execcmd.GracePeriod is exported so the watchdog delay is visibly derived from the kill schedule.

New tests cover both behaviors: TestExecuteQueryInternal_PipesHeldOpenAfterCancel simulates an orphaned descendant holding the pipes (hangs without the watchdog), and TestExecuteQueryInternal_DrainsStreamsBeforeWait has the mock Wait emulate exec's pipe close on return (truncates stdout/stderr under the old ordering). Both were validated by reintroducing each bug and watching the test fail.

Fixes #134.