Fix shared-memory frame size guard

[fallintoplace]

Summary

Reject frames that exceed a shared-memory slot before claiming or writing to the ring buffer. The error includes the frame dimensions, pixel format, byte count, and configured slot capacity.

This also adds a defensive read-side check for oversized FrameSignal.data_sz values, and covers the failure mode with focused regression tests that verify rejected frames do not advance the writer or corrupt the next slot header.

Validation

• PYTHONPATH=server-runtime:utils/xr-ai-logging uv run --python 3.12 --project agent-sdk --with pytest --with pytest-asyncio --with loguru pytest tests/test_shm_ring_buffer.py
• PYTHONPATH=server-runtime:utils/xr-ai-logging TMPDIR=/tmp uv run --python 3.12 --project agent-sdk --with pytest --with pytest-asyncio --with loguru pytest --basetemp=/tmp/xr-ai-pytest tests/test_participant_events.py
• uvx ruff check agent-sdk/xr_ai_agent/_shm.py tests/test_shm_ring_buffer.py
• git diff --check