Add task startup overrun warning to ThreadPoolTaskScheduler

[itsmehotpants]

What this PR does

Closes #33856

Adds a configurable taskStartupOverrunThreshold property to ThreadPoolTaskScheduler. When set, the scheduler emits a WARN-level log message whenever a scheduled task starts significantly later than its intended fire time, making it straightforward to diagnose thread starvation caused by a too-small thread pool or long-running tasks holding scheduler threads.

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Motivation

A common Spring Boot gotcha: spring.task.scheduling.pool.size defaults to 1. If any scheduled task runs long, all subsequent tasks queue up behind it and start late — sometimes by seconds or minutes. Currently this is silent; there is no built-in diagnostic. Users discover the problem only after significant delay or by reading the documentation carefully.

This PR introduces an opt-in warning threshold so the issue surfaces immediately in logs.

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Changes

ThreadPoolTaskScheduler

Addition	Detail
taskStartupOverrunThreshold field	volatile @Nullable Duration, null by default (warnings disabled unless opted in)
setTaskStartupOverrunThreshold(Duration)	Setter with full Javadoc; accepts null to disable
getTaskStartupOverrunThreshold()	Accessor
beforeExecute(Thread, Runnable) override	Checks RunnableScheduledFuture.getDelay(NANOSECONDS); logs WARN when overrun ≥ threshold

The hook uses the already-wired beforeExecute callback that the anonymous ScheduledThreadPoolExecutor subclass created in createExecutor() delegates to, so no changes to the executor creation logic are needed.

ThreadPoolTaskSchedulerOverrunTests (new)

7 JUnit 5 tests covering:

• Default threshold is null
• Getter/setter round-trip
• Reset to null
• Normal execution with threshold set
• Normal execution without threshold
• Overrun detection under thread starvation (pool size 1, blocking first task)
• No spurious warnings when threads are sufficient (pool size 2)

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Usage

// Via API
ThreadPoolTaskScheduler scheduler = new ThreadPoolTaskScheduler();
scheduler.setPoolSize(1);
scheduler.setTaskStartupOverrunThreshold(Duration.ofSeconds(1));
scheduler.initialize();

// Or via Spring Boot configuration (with a custom customizer)

When a task starts 1 450 ms after its scheduled time:

WARN --- ThreadPoolTaskScheduler : Task [org.springframework.scheduling...] is starting 1450ms late
         (threshold: 1000ms). Consider increasing the thread pool size or reducing task duration.

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Checklist

• Unassigned issue with no maintainer claim
• Opt-in by default (null threshold = no change in behaviour for existing users)
• Uses the inherited logger from ExecutorConfigurationSupport (no new dependency)
• 7 integration tests
• Full Javadoc on all new public API