fix: preserve collected data when controller script is interrupted by signal

[harp-intel]

Summary

• When SIGINT is sent during flamegraph/telemetry collection on a remote target, the controller script handles the signal gracefully and outputs all collected data before exiting with code 0
• However, perfspect's signal handler then sends SIGINT to its child SSH process, causing SSH to return exit code 255 instead of relaying the controller's exit code
• Previously, any non-zero controller exit code caused all collected data to be discarded
• Now, when the exit code is non-zero but stdout contains valid controller output (the <----------------------> delimiter from print_summary), the data is parsed and returned with a warning instead of being treated as a fatal error

Test plan

• Run flamegraph collection on a remote target and send SIGINT during collection — verify output files are generated
• Run flamegraph collection normally (no SIGINT) — verify no change in behavior
• Run make test — all unit tests pass

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