A non-append SelectRoutes() wipes the current selection before applying
the requested one, but it validated the requested IDs only afterwards,
while already mutating. A request naming no available route at all left
every route deselected and returned an error - so a typo in a route ID
silently dropped the user's exit node, and the routes stayed applied
while the selector claimed nothing was selected.
Validate first and bail out before touching any state when nothing in
the request is available. A request with at least one available route
keeps applying the valid part and reporting the rest, and an empty
request still deselects everything, since that is the caller asking for
exactly that rather than a failed lookup.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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