[Backport 7.80.x] [DBMON-6602] Avoid cleanup when cancel called while check running

[dd-octo-sts]

Backport 4c7e8bb from #23728.

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What does this PR do?

Fixes a race condition where cancel() could destroy check state (close database connections, null _query_manager, _db, etc.) while check() is still running on another thread. This caused SIGSEGV crashes in libpq during cluster check rebalancing when multiple Postgres checks were unscheduled simultaneously.

The fix splits cancel into two phases:

• Signal phase (_cancel_async_jobs): sets cancel events on async job threads. Safe to run concurrently with check().
• Finalize phase (_finalize): joins async job threads, closes connections, and nulls state. Only runs when check() is guaranteed idle.

A lock coordinates run() and cancel() to ensure _finalize() executes exactly once — either by cancel() directly (if the check is idle) or by run()'s finally block (if the check is in-flight).

The cleanup introduced in #23640 is required — it breaks reference cycles (check → async job → check, check → query manager → check, check → logger → check) and closes connections so that garbage collection can free the check object in a timely manner. The problem was not the cleanup itself but where it ran: directly inside cancel(), which can execute concurrently with check(). This PR preserves all of that cleanup by moving it to _finalize(), which only runs when check() is guaranteed idle.

Note: the base AgentCheck class currently lacks a formalized pattern for coordinating cancel() with an in-flight run(). The run()/cancel() lock coordination is implemented directly in the Postgres check for now, with the intention of moving this into the base class (and potentially the Go-side CheckWrapper) so all checks benefit from the same safety guarantees.

Motivation

The base class documents that cancel() "can be called while the check is running." PR #23640 added aggressive cleanup to cancel() (closing connections, nulling attributes) for GC improvements, which violated this contract. When cancel() ran while check() was mid-query via psycopg, _close_db() freed the underlying libpq socket. When the I/O completed and check() resumed, libpq dereferenced freed memory, producing SIGSEGV addr=0x0 instead of a recoverable Python exception. This was observed during a cluster check rebalance (26 configs removed, 29 added) where 8 Postgres checks were cancelled in rapid succession.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[dd-octo-sts]

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❌ Patch coverage is 94.23077% with 6 lines in your changes missing coverage. Please review.
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• Overall Coverage: 93.77%

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