feat(auditlog_ng): update v2 auditevent schema to event catalog 2.4.0

[mtsvetanov071]

Summary

• Update auditevent.proto from telemetry-semantic-conventions 2.4.0 (adds Turn events, Memory access events, and operational field cleanup per e840153)
• Regenerate Python models (auditevent_pb2.py, auditevent_pb2.pyi) with buf generate
• Add newly generated buf/validate/ package (protovalidate stubs, new in 2.4.0)
• Bump version 0.56.0 → 0.57.0 and update uv.lock

Test plan

• Verify buf generate produces identical output locally
• Run existing auditlog_ng tests
• Confirm new AI Turn / Memory access event types are importable from the generated package

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