Make agent suspend tests release on the suspend signal

The single- and multi-level suspend/restore tests gated their slow
leaf tool on a manual release channel closed 50ms after cancel().
That sleep was a guess at how long the suspend signal takes to reach
the running sub-agent, so the post-tool turn boundary could observe
the release before cancellation and complete the run instead of
checkpointing, making the assertions timing-dependent.

Expose the per-run suspend signal through SuspendSignalFrom in an
export_test shim and have the leaf tools block on it directly. The
tool now returns only once the graceful-suspend signal has actually
propagated to its agent, so suspension is observed deterministically
without sleeps or release channels.

Signed-off-by: Bryan Frimin <bryan@probo.com>

pkg/agent/export_test.go

@@ -0,0 +1,21 @@
+// Copyright (c) 2026 Probo Inc <hello@getprobo.com>.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+// OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+package agent
+
+// SuspendSignalFrom exposes the per-run graceful-suspend signal carried in
+// ctx to external tests. Tests use it to block in-flight leaf tools until
+// the suspend signal has actually reached the running (sub-)agent, making
+// suspend/restore assertions deterministic instead of timing-dependent.
+var SuspendSignalFrom = suspendSignalFrom