fix(realtime): wait for subscribe ack before returning

[Dodothereal]

Summary

• make AsyncRealtimeChannel.subscribe() wait for a definitive subscribe outcome before returning
• capture subscribe outcomes (SUBSCRIBED, CHANNEL_ERROR, TIMED_OUT) in an internal future
• propagate callback exceptions through await subscribe(...) instead of returning early
• add unit tests verifying: (1) subscribe does not return before ack, and (2) callback exceptions are raised to caller

Why

Issue #1209 reports await channel.subscribe(callback) returning before the server reply is handled, causing race conditions. This change ensures await semantics match subscription acknowledgement lifecycle.

Testing

• uv run --package realtime pytest tests/test_channel.py tests/test_timer.py (run in src/realtime)
• make realtime.mypy
• uv run ruff check src/realtime/src/realtime/_async/channel.py src/realtime/tests/test_channel.py

Closes #1209

[Dodothereal]

@olirice @silentworks could you please review this bugfix for #1209 when you have time? Thanks.

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[o-santi]

Hey @Dodothereal, thanks for the PR. I'm afraid we cannot merge this as of now because it would be breaking for most clients. People write their clients assuming that it doesnt block nowadays, so their code would be most likely wrong after this.

I intend to fix this problem, together with some other ones in realtime, in the next v3 version (#1370). Feel free to ask any other improvement that may be a breaking change there too.