EXP-1171: Raise connection error when beanstalkd closes a socket

[Steven Roomberg (sroomberg-ep)]

Summary

• classify zero-byte socket reads as BeanstalkConnectionError instead of empty protocol errors
• apply EOF handling consistently across response headers, payloads, names, and status words
• bump the package patch version from 0.9.0 to 0.9.1

Testing

• venv/bin/pytest -q tests/unit (25 passed)
• just lint
• just mypy
• just build
• Full suite on Bigdev with native beanstalkd (49 passed)
• Sysslack failover test: startup with one unavailable node, node recovery, established connection closure, and continued job processing on the surviving node

Sentry was disabled for the Sysslack integration run with EASYFLASK_DISABLE_SENTRY=1.