fix: reduce sandbox sink buffering and spool timeouts

[2830500285]

Summary

• keep WorkspaceJsonlSink memory bounded to pending events by reading the existing workspace outbox during flush and clearing pending bytes after a successful write
• preserve existing workspace JSONL contents across repeated flushes without duplicating lines
• spool HttpProxySink events when urlopen raises a direct timeout/OSError delivery failure

Test plan

• uv run pytest tests/sandbox/test_session_sinks.py -q -k "clears_flushed_buffer or spools_direct_timeout"
• uv run pytest tests/sandbox/test_session_sinks.py -q
• make sync
• UV_PROJECT_ENVIRONMENT=.venv_314 make sync
• UV_PROJECT_ENVIRONMENT=.venv_314 bash .agents/skills/code-change-verification/scripts/run.sh

Issue number

Closes #3640
Closes #3641

Checks

• I have added new tests (if relevant)
• I have added/updated the relevant documentation
• I have run make lint and make format
• I have made sure tests pass