TryInvokeAsync swallowed all exceptions including HubException, so a
blocked merge (uncommitted changes in target, mid-merge state, inactive
worktree) returned null silently — the task stayed WaitingForReview with
no feedback shown. Switch to a direct _hub.InvokeAsync so both VM
catch blocks (TasksIsland ErrorReported, DetailsIsland ShowErrorAsync)
actually fire.
Add regression tests for both call sites verifying that a throwing
worker causes the error to be reported.
