fix: clean up auth when deleting named contexts

[cotishq]

Description

• Fixes named context deletion so cleanup uses the removed context's referenced user and server values instead of the context name.
• Removes the associated auth entry when deleting a context, matching the expected cleanup behavior for credentials tied to that server.
• Adds a regression test covering context --delete for a named context where contexts[].name differs from contexts[].user.

Screenshot

Related issue(s)

Fixes #427