fix: reduce LongBridge buy-skip alert noise

[Pigbibi]

Summary

• keep cash-reserve-only buy deferrals silent instead of surfacing noisy skip alerts
• show account cash and investable cash in LongBridge heartbeats
• keep explicit notes only when broker buying power still looks blocked by open orders, settlement, or similar holds

Verification

• python3 -m py_compile notifications/telegram.py application/rebalance_service.py tests/test_rebalance_service.py
• python3 -m ruff check notifications/telegram.py application/rebalance_service.py tests/test_rebalance_service.py
• PYTHONPATH=.:/Users/lisiyi/Projects/QuantPlatformKit/src:/Users/lisiyi/Projects/UsEquityStrategies/src python3 -m unittest tests.test_rebalance_service -v