Separate transport and status availability

[mel9320107]

What changed

• Track valid transport traffic separately from fresh spa status frames.
• Expire status availability after 10 seconds without a regular status update, even if other RS-485 traffic continues.
• Prevent state-derived entities and commands from relying on retained stale spa state.
• Keep state-independent diagnostic requests available while transport traffic remains healthy.
• Expose distinct transport/status timestamps and availability in diagnostics.
• Add focused tests for transport-only traffic, status freshness expiry, and disconnect behaviour.

Why

Previously, any recognised protocol frame refreshed the integration's single availability flag. Bus-management or configuration traffic could therefore make an old SpaState appear current after a reconnect or after regular status frames stopped arriving.

Validation

• python -m unittest -q tests.test_client_availability — 3 tests passed.
• python -m compileall -q custom_components/spa_pool tests — passed.
• Remote comparison confirms this branch is based directly on main and changes only the six integration files plus the focused test file.

Hardware testing requested

Please install this draft branch in Home Assistant and verify normal entity availability, command behaviour, and recovery after temporarily interrupting spa status traffic or the TCP connection.