Rewrite the WithStopSignal-driven test in restore_test.go to use a
cancellable ctx. Update agent_test.go's "context cancellation"
case from asserting "cannot complete" failure to asserting a
SuspendedError. Add cancel_test.go covering both pre-first-turn
cancel (no LLM call, empty checkpoint persisted) and mid-run
cancel from inside a tool (just-completed turn preserved in the
checkpoint, second LLM call suppressed).
Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>
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