fix(scheduler): improve async task lifetime safety

[ReallocAll]

Improves async scheduler task lifetime safety during plugin cancellation and reload.

• Prevents queued and running async callbacks from outliving C++ plugins
• Waits for async work to finish before unloading plugin libraries
• Fixes async task cancellation, exception cleanup, and running-state reporting
• Makes internal task scheduling safe across BDS worker threads
• Handles executor enqueue failures without leaving pending task counts stuck
• Adds regression tests for async cancellation and cross-thread scheduling

Follow-up to #351.

[Copilot]

Pull request overview

This PR strengthens async scheduler task lifetime safety during plugin cancellation/reload, aiming to prevent async callbacks from outliving unloaded C++ plugin libraries (follow-up to #351).

Changes:

• Adds async-task draining on plugin clear/unload (waitForAsyncTasks) and improves cancellation cleanup to release plugin callback references promptly.
• Improves thread-safety/robustness in scheduling (atomic tick tracking, safe task capture into executor, corrected async running-state reporting).
• Extends test coverage for async cancellation, cross-thread scheduling, and executor waiting behavior.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tests/test_thread_pool_executor.cpp	Adds coverage for zero-thread construction behavior and executor wait() semantics.
tests/test_scheduler.cpp	Adds regression tests for cross-thread scheduling, async lifecycle/cancellation, and exception recovery.
src/endstone/core/scheduler/thread_pool_executor.h	Tracks pending tasks and adds wait() to support draining queued async work.
src/endstone/core/scheduler/thread_pool_executor.cpp	Ensures at least one worker thread, implements wait(), and updates worker bookkeeping.
src/endstone/core/scheduler/task.h	Initializes plugin_ to nullptr for safer default state.
src/endstone/core/scheduler/scheduler.h	Adds waitForAsyncTasks() and makes current_tick_ atomic.
src/endstone/core/scheduler/scheduler.cpp	Improves cancellation cleanup, async running-state, cross-thread safety, and adds async wait support.
src/endstone/core/scheduler/async_task.h	Adds a condition variable and wait() for async task completion.
src/endstone/core/scheduler/async_task.cpp	Makes cancellation/start bookkeeping safer and adds task completion signaling.
src/endstone/core/plugin/plugin_manager.cpp	Waits for async work to finish after disabling plugins and before clearing/unloading them.

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