Keep subscribe_with_handler running until the caller closes it

[CasperGN]

Description

Based on #1231. Merge it after #1231. Until #1231 merges, this diff also shows #1231's commits; the changes of its own are the last two commits (0844c95, 942c58d). It reuses _closed, _close_stream(), the per-stream send queue and the fake sidecar hooks from #1231.

subscribe_with_handler could stop delivering messages without any error. This PR fixes it in both clients.

• Sync, retry path: after a failed reconnect the loop slept 5 seconds and then exited. It now retries every SUBSCRIPTION_RECONNECT_BACKOFF_SECONDS (5) until the close function is called. The close function also ends a loop that is waiting to retry, so it returns right away.
• Both, pause after a stream error: when next_message() gives up (after its own bounded reconnects from Return the next message after a streaming subscription reconnects #1231) or raises a non-retryable error, the loop now waits SUBSCRIPTION_RECONNECT_BACKOFF_SECONDS before reconnecting. Before, a stream that kept failing right after connecting was reconnected with no pause, forever. Closing ends the wait at once.
• Both, handler errors: an exception raised by the handler was treated as a stream error. The stream was reconnected and the message was delivered again. Now the exception is logged with its traceback, the message gets a retry response, and the next message is read. The stream stays open.
• Async, errors ended the task: any error other than StreamInactiveError ended the handler task, and it only showed up later as "Task exception was never retrieved". The task now follows the same rules as the sync loop. It reconnects on stream errors, retries after a failed reconnect, and stops only after the caller has closed the subscription. grpc.aio raises asyncio.CancelledError when reading a stream that close() cancelled. That counts as a normal exit only after a close, and is re-raised otherwise.
• Async, task reference: the SDK now keeps a reference to the handler task, so it can't be garbage-collected while it runs.
• Async, close waits: the close function now waits up to SUBSCRIPTION_CLOSE_TIMEOUT_SECONDS for the handler task to finish. It skips the wait when it is called from inside the handler.

Tests:

• The async test_subscribe_topic_with_handler is restored.
• New tests for both clients cover:
  • a reconnect that fails and then succeeds
  • a handler that raises once
  • a stream that fails with CANCELLED or NOT_FOUND
  • the pause after a stream error
  • closing while the loop is waiting
• The async tests also check that the task survives garbage collection.
• Return the next message after a streaming subscription reconnects #1231's test_subscribe_topic_with_handler_currently_reconnects_after_handler_error documented the old handler-error behaviour. It is replaced by a test that expects a retry response.

Issue reference

Please reference the issue this PR will close: Fixes #1233

Checklist

• Code compiles correctly
• Created/updated tests
• Extended the documentation

RELEASE NOTE: FIX subscribe_with_handler keeps retrying while the sidecar is unavailable, pauses between reconnects, does not reconnect on handler errors, and the async version no longer stops silently.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 98.52941% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.38%. Comparing base (fb229bc) to head (942c58d).

Files with missing lines	Patch %	Lines
dapr/clients/grpc/subscription.py	96.87%	2 Missing ⚠️
dapr/aio/clients/grpc/subscription.py	98.59%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1235      +/-   ##
==========================================
+ Coverage   83.89%   84.38%   +0.48%     
==========================================
  Files         123      123              
  Lines       10265    10400     +135     
==========================================
+ Hits         8612     8776     +164     
+ Misses       1653     1624      -29     

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