Log failures currently swallowed by external preview lifecycle operations

## Summary
Audit external-preview operations for exceptions that are currently swallowed or otherwise not observable, then add useful error logging without changing the user-facing recovery policy.

## Rationale
Failures in opening, closing, or switching external previews need diagnostics so maintainers can identify plugin and integration problems. This work should remain separable from behavioral recovery changes.

## Affected area
- `Flow.Launcher/ViewModel/MainViewModel.cs`
- External-preview methods in `PluginManager` and their plugin interfaces

## Required changes
- Identify exception paths for external-preview open, close, and switch operations that can be swallowed or lost by fire-and-forget call sites.
- Add structured, contextual error logging at the appropriate boundary.
- Ensure failure does not incorrectly update external-preview state.
- Add focused tests for logging/state behavior where practical.

## Acceptance criteria
- Failures from external-preview lifecycle operations are observable in Flow Launcher logs with enough context to diagnose the failed operation.
- Exceptions are not silently swallowed.
- Logging changes do not alter successful preview behavior.

Requested by @DavidGBrett.

Related PR: https://github.com/Flow-Launcher/Flow.Launcher/pull/4529
See PR comment: https://github.com/Flow-Launcher/Flow.Launcher/pull/4529#pullrequestreview-4821854295