The cli tests that launch pi deleted only the session login names, so pi's
preflight passed or refused by whatever provider keys the host exported. A
shared helper now deletes every credential pi's preflight reads and gives pi
launches one made-up key. The refusal itself is unchanged.
pi bundle() takes a default argument again, so a call with none returns the
non-proxy bundle, as before P3. pi.bundle leaves out a launch.models entry
whose provider has no proxy route instead of failing every pi launch (F2);
malformed proxy entries still refuse. The invalid session proxy context
BundleError gets the code invalid-proxy-context. New tests cover F2, the code,
and the three proxy account branches in the launcher (F3).
jason.woltje