fix: resolve COM memory instability and ensure graceful Excel teardown

[eli-lindner-sj]

This PR implements a scope-isolated cleanup pattern to ensure reliable Excel lifecycle management:

Key Changes:

• Scope Isolation: Refactored ExcelWorker.Run() to move core processing into a nested ExecuteRunLoop. This ensures that all local COM references naturally go out of scope and become eligible for collection before the "double-tap" GC collection (GC.Collect() + GC.WaitForPendingFinalizers()) is triggered in the finally block.
• Removed Manual Release: Eliminated all calls to Marshal.FinalReleaseComObject in favor of the more stable scope-isolated GC pattern, preventing DLR-related interop exceptions.
• Process Abstraction: Introduced IProcessInterop to abstract OS-level process interactions. This allowed for the replacement of "hollow" tests with deterministic unit tests that verify tracking and fallback-kill logic without requiring a real Excel instance.
• Resilient PID Tracking: Improved GetExcelProcessId to handle failures gracefully, ensuring that valid PIDs are always tracked for cleanup even if initial resolution is delayed.