[Service Bus] aio AutoLockRenewer: prune completed futures to fix memory leak

[EldertGrootenboer]

Summary

Fixes #48366.

The async AutoLockRenewer appended each renewal asyncio.Future to an internal list _futures in register() and never removed it on completion — the list was drained only by close(). A long-lived renewer (the documented one-renewer-per-app pattern) therefore accumulated one future per registered message for its whole lifetime: memory grew proportional to the total messages processed, reclaimed only on close().

Fix

• _futures is now a set, and register() registers renew_future.add_done_callback(self._futures.discard) so each future is removed the moment its renewal coroutine completes (success, timeout, or error). The collection stays bounded by the number of active renewals rather than the all-time count of register() calls.
• close() is unchanged (await asyncio.wait(self._futures) still awaits any remaining active futures; asyncio.wait snapshots its input, so callback-driven removal during the await is safe).
• No public API change — private state, a done-callback, and a typing import.

Tests

New tests/unittests/test_auto_lock_renewer_futures.py (offline, no network):

• bounded-after-settle: registered renewals complete and leave the set empty (fails against the unfixed code)
• close() still awaits an active renewal
• an errored renewal is still pruned
• over-prune guard: active futures are retained while completed ones are removed

The full offline unit suite passes. The Python sync renewer is unaffected (it self-drains via a queue.Queue), and the .NET/Java/JS SDKs prune renewal handles on completion, so this is an aio-only fix.

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The implementation safely bounds retained futures and includes focused coverage for completion and shutdown behavior.

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Pull request overview

Fixes #48366 by pruning completed async lock-renewal futures, preventing unbounded memory growth.

Changes:

• Tracks active futures in a set and removes them on completion.
• Adds offline regression tests for completion, errors, and shutdown.
• Documents the fix in the changelog.

File summaries

File	Description
_async_auto_lock_renewer.py	Prunes completed renewal futures.
test_auto_lock_renewer_futures.py	Adds regression coverage.
CHANGELOG.md	Records the memory-leak fix.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0
• Review effort level: Balanced

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