Tests the checkpoint persistence and supervisor lifecycle against a
real Postgres database:
- PGCheckpointStore Save/Load/Delete round-trip
- Supervisor claims PENDING run and completes it
- Cooperative stop/resume via stop_requested flag
- SIGTERM battle test: 3 kill/resume cycles across 10 tool-call
turns with parallel calls, long-running tools, thinking text,
and progressive checkpoint accumulation
Tests skip gracefully when Postgres is unavailable.
Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>
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