fix(subagent): preserve interruption state and prevent duplicate delegation

[xiami762]

Summary

• report interrupted subagent runs as failed terminal results while preserving the child session ID
• propagate the interrupted status to parent tool metadata so the child session can be resumed
• preserve the latest running tool metadata when direct task cancellation writes the terminal error state
• drain foreground parallel tool calls before propagating provider stream errors or cancellation
• suppress step replay after tool execution has started to prevent duplicate delegation

Testing

• uv run pytest -q tests/tool/test_delegate_task_compat.py tests/tool/test_subagent_hooks.py tests/session/test_runner_llm_hooks.py tests/session/test_stream_processor_parallel_tools.py tests/session/test_auto_model_failover.py tests/session/test_runner_step.py tests/session/test_stream_processor.py
• uv run ruff check flocks/session/streaming/stream_processor.py flocks/tool/subagent_result.py flocks/tool/agent/delegate_task.py flocks/session/runner.py tests/session/test_stream_processor.py tests/tool/test_delegate_task_compat.py tests/session/test_runner_llm_hooks.py tests/session/test_runner_step.py
• git diff --check

Result: 239 tests passed; Ruff and diff checks passed.