Feature: Invoke markitdown via external process (Phase 2 of #60)

[EmbarrassingMoment]

Summary

Build on the Phase 1 settings + environment helpers added for #60 by actually invoking markitdown as an external process and capturing its output.

Scope

• Add a converter class (e.g. FMarkitdownConverter) that runs the resolved executable using FInteractiveProcess / FMonitoredProcess
• Asynchronous invocation with progress / cancel support
• Capture stdout/stderr for diagnostics, surface failures via FMessageDialog + UE_LOG
• Detect missing executable up front and present an actionable error pointing to the Markitdown settings page
• No UI / no import factory wiring yet — those land in their own issues

Out of scope

• Import Factory extensions (tracked separately)
• Editor Utility Widget UI (tracked separately)

References

• Parent: Feature: Add "Convert to Markdown" Editor Utility Widget using markitdown (Python) #60
• Phase 1 commit: feat: add Markitdown settings and environment helpers