Fix long-response fallback in Rich streaming

[tsint]

Summary

Fix Rich Live streaming fallback for long responses.

Problem

Large streamed responses can exceed the practical redraw area for Rich Live. In some terminals, this may leave stale or duplicated output while the response is still streaming. The previous fallback logic also did not fully isolate fallback mode to the current response.

Changes

• Added per-response plain streaming fallback state.
• Estimate rendered line count with Rich instead of relying only on newline counts.
• Make the Live line limit aware of terminal height.
• Stop and clear the active Live frame before falling back to plain output.
• Clear the accumulated buffer after fallback to avoid duplicate output.
• Reset fallback state in flush_response() so the next response can use Live again.
• Added focused tests for normal Live streaming, first-chunk fallback, and active-Live fallback.

Test Plan

uv run pytest -q tests/test_render_streaming.py