The payload builder filled anything matching the validator's broad
credential pattern that wasn't a non-empty string, so RCON_PORT,
ENABLE_RCON and ARK's RCONEnabled/RCONPort became 'boot-123456' in the
first runner run. Only fill keys named like passwords (or x-generate),
and only when missing or an empty string. Adds a test.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
