JVNAUTOSCI-1202: enforce completion gate on no-tool and validation-error paths

[witbrock]

Summary

• route tool_calling.plan direct-response fallback through postcondition_critic instead of terminating directly
• route tool_calling.validate validation-error fallback through postcondition_critic instead of terminating directly
• derive terminal /von/generate progress success and completion_gate_* fields from the latest turn_completion_gate auxiliary call
• align orchestrator trace terminal status with completion-gate outcome (completed vs follow_up_required)

Why

This closes a false-completion gap where mutation-required turns could complete without critic/gate evaluation, and prevents stale terminal progress signals from reporting success when follow-up is required.

Tests

• pdm run pytest tests/backend/test_turn_execution_workflow_definitions.py tests/backend/test_orchestrator_turn_execution_gate.py tests/backend/test_tool_progress_liveness.py -q
• pdm run pytest tests/backend/test_von_generate_rag_trace.py tests/backend/test_von_generate_llm_debug_parse_error.py -q
• pdm run pytest tests/backend/test_orchestrator_durable_instance_telemetry.py tests/backend/test_orchestrator_turn_execution_gate.py -q
• pdm run pyright <changed python files>