[Enhancement][Python] Make PushConsumer message reception non-blocking to support large numbers of topics

### Before Creating the Enhancement Request

- [x] I have confirmed that this should be classified as an enhancement rather than a bug/feature.

### Programming Language of the Client

Python

### Summary

The Python PushConsumer submits a blocking receive call to a ThreadPoolExecutor, so every in-flight long-polling request occupies one worker until the server-stream completes.

Concurrent reception is therefore bounded by the pool size rather than by the number of assigned queues, and with many subscribed topics most queues wait many long-polling windows before their first receive — appearing effectively starved in production. The log repeats:
> 2026-08-26 11:44:16,321 [WARNING] process queue is idle, idle_duration: 1022, max_idle_duration: 69, after_cache_full_duration: 1787715857, mq: broker-0.Topic100.-1
2026-08-26 11:44:16,332 [WARNING] process queue is idle, idle_duration: 1022, max_idle_duration: 69, after_cache_full_duration: 1787715857, mq: broker-0.Topic100.-1
2026-08-26 11:44:16,344 [WARNING] process queue is idle, idle_duration: 1022, max_idle_duration: 69, after_cache_full_duration: 1787715857, mq: broker-0.Topic100.-1
2026-08-26 11:44:16,355 [WARNING] process queue is idle, idle_duration: 1022, max_idle_duration: 69, after_cache_full_duration: 1787715857, mq: broker-0.Topic100.-1
2026-08-26 11:44:16,366 [WARNING] process queue is idle, idle_duration: 1022, max_idle_duration: 69, after_cache_full_duration: 1787715857, mq: broker-0.Topic100.-1
2026-08-26 11:44:16,377 [WARNING] process queue is idle, idle_duration: 1022, max_idle_duration: 69, after_cache_full_duration: 1787715857, mq: broker-0.Topic100.-1

### Motivation

When a single PushConsumer subscribes to 100 topics, messages on most of those topics are effectively never consumed.
The consumer stays up and healthy and a few topics work normally, but the rest produce no deliveries for as long as the process runs — only the process queue is idle warning above, repeating indefinitely.

### Describe the Solution You'd Like

**Make reception non-blocking**. Replace the unbounded-default reception pool with a fully callback-driven receive path that does not occupy one executor worker per in-flight receive.

**Fix the related defects that keep starved queues from recovering**. These sit on the same paths and are best fixed together:
- The expired-queue cleanup has an inverted condition.
- The assignment handler starts reception for all existing queues rather than only the newly created ones.
- The non-FIFO consumption path misses a return after discarding a corrupted message.
- The FIFO discard path also lacks a return after its recursive retry.

### Describe Alternatives You've Considered

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### Additional Context

_No response_

Start at the Python PushConsumer receive path and its ThreadPoolExecutor usage, then trace queue assignment and the FIFO and non-FIFO discard paths. Done means reception no longer starves queues as subscribed topics increase and the expired-queue, assignment, and discard-path defects are corrected without breaking message delivery.

Written by the indexing model from the issue text.