Streaming stops after the first ping/pong (flags freeze 10s after start since 1.1.8) #18

Summary

Since 1.1.8, the streaming data source stops permanently ~10 seconds after start. Flag changes are no longer received until the process restarts, while client.initialize still returns True.

Confirmed root cause

• The SDK sends {"messageType":"ping"} every 10 s; the evaluation server replies {"messageType":"pong","data":{}} (PingMessageHandler.cs, unchanged since 2023).
• Since feat: expose variation ID, SDK status, and harden runtime safety #14, Streaming._on_message raises ValueError('invalid streaming data') for any message rejected by valide_all_data(), which only accepts data-sync.
• The except branch closes the socket with _SelfClosed(is_reconn=False), so the streaming thread exits and never reconnects.

Reproduced on 1.1.9 with a clean install against FeatBit v6.0.0:

t=0s : toggled to False -> client sees False | state=OK
t=12s: toggled to True  -> client sees False | state=OFF   (log: close status: 1001 / self close)

On CPython 3.13/3.14, pip/Poetry can only install 1.1.9 (1.1.7 declares <3.13), so there is no working version there.

Proposed scope

• Treat pong as a keep-alive reply in _on_message and return before data validation.
• Keep the existing behaviour for genuinely invalid data unchanged.
• Add unit tests covering a pong frame and the existing invalid-data path.

Acceptance criteria

• A pong frame does not close the connection or change the data-update state.
• Invalid non-pong messages still close the stream with the data-invalid state.
• The full existing suite passes.
• Flag changes are received more than 10 s after start against a live evaluation server.