Add native Assist streaming for OpenClaw conversation

[lukcz]

Summary

• migrate the OpenClaw conversation agent to an entity-backed streaming agent
• feed OpenClaw SSE deltas into Home Assistant ChatLog and build the final result via HA helpers
• keep the existing automation event, session resolution, prompts, heuristics, and add fallback plus tests

Testing

• pytest -q tests/test_conversation_streaming.py

[lukcz]

Superseded by #21, which contains the native Assist streaming change only. This follow-up PR drops the later thinking/debug additions so the review can focus on the minimal Home Assistant streaming integration.