Add task based execute

[mkwired]

Summary

Adds task-based Octave command execution with cancellation support and hardens process lifecycle handling around errors, timeouts, disposal, and pooled worker reuse.

Changes

• Added OctaveContext.Execute(string command, CancellationToken cancellationToken) for asynchronous command execution.
• Serialized sync and async execution per context to avoid overlapping writes to the same Octave REPL process.
• Added cancellation and disposal handling that interrupts or retires active workers instead of returning unsafe processes to the pool.
• Improved command completion detection by waiting for both stdout and stderr command-boundary markers.
• Retired workers after stderr errors, timeouts, process exits, cancellation, and failed writes so later executions can recover with a fresh process.
• Reworked ObjectPool<T> to use SemaphoreSlim for concurrency limits and added explicit disposal/idle reclaim cleanup.
• Updated tests for async execution, cancellation, disposal races, process
• dotnet build src/Octave.NET/Octave.NET.csproj -f netstandard2.0 --no-restore
• dotnet build src/Octave.NET/Octave.NET.csproj -f net10.0 --no-restore