Fix launch canKill inversion and tray bugs

[SimonCropp]

• DiffRunner: the sync launch path passed tool.IsMdi instead of !tool.IsMdi as canKill on the already-running and max-instance paths, diverging from the async path. For MDI tools (VS Code, Cursor, Visual Studio, Araxis) this could make the tray kill the shared editor when a diff is accepted/discarded; for non-MDI tools it left diff windows orphaned. Compute canKill once so the paths can't drift.
• Program.ReBindKeys: the "Accept Open" hotkey was registered under KeyBindingIds.AcceptAll, so configuring both it and "Accept All" silently disabled "Accept All". Route bindings through a single BuildKeyBindings mapping with the correct id.
• SolutionDirectoryFinder: the cached-prefix match had no directory-separator boundary, so a file under "AppTests" resolved to a cached "App" solution. Add a boundary check, make it case-insensitive, and prefer the nearest enclosing directory.
• PiperClient: the async sends accepted a cancellation token but never used it. Thread it through connect/write and let cancellation surface to the caller instead of being logged as a send failure.
• ProcessEx.KillAndDispose: the error handlers read Id/MainModule after the kill, which can throw and escape the method. Capture the process description up front.
• FileEx.SafeDeleteDirectory threw on a temp directory that contained only empty sub-directories (non-recursive delete). Delete recursively and rename the misleading IsEmptyDirectory (which only checked for files) to ContainsFiles.

Add regression tests for each.