fix: guard zero-target data sampling

[rupurt]

Summary

• guard realtime data sampling when max_bars_per_timeframe produces a zero target
• preserve the latest bar for single-bar buffer limits
• add a regression test for single-bar sampling

Tests

• uv run pytest tests/realtime_data_manager/test_memory_management.py
• uv run pytest tests/realtime_data_manager/test_memory_management.py tests/realtime_data_manager/test_data_processing.py
• uv run ruff check src/project_x_py/realtime_data_manager/memory_management.py tests/realtime_data_manager/test_memory_management.py
• uv run mypy src/

Note: full uv run ruff check . still reports pre-existing unrelated lint in scripts and old tests.