Fix thread pool worker accounting drift in ThreadPoolExecutor

[BoBiene]

Motivation

• Prevent _freeThreads counter from drifting under contention by ensuring it is only adjusted while a worker is actually waiting.
• Avoid desynchronization between scheduling and worker state updates that could cause incorrect pool sizing decisions.
• Ensure thread lifecycle bookkeeping remains consistent by cleaning up _runningThreads and _pool when a worker exits.

Description

• Removed the eager decrement of _freeThreads in InternalExecute so the free-thread count is not manually manipulated when enqueuing tasks.
• Rewrote RunPoolThread wait loop so a worker increments _freeThreads only while inside Monitor.Wait and always decrements it in a finally block, and the worker dequeues and runs tasks after waking.
• Added an outer try/finally in RunPoolThread to decrement _runningThreads and remove the finished thread from _pool when a worker terminates, preserving consistent pool state.
• Preserved the existing suppression of unexpected exceptions inside the worker loop to avoid breaking shutdown semantics.

Testing

• Attempted to run dotnet test SharpCifs.SMBv1.sln, but the test run failed in this environment because dotnet is not installed.
• No other automated tests were executed in this environment due to the missing runtime.

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Codex Task