Raise a specific XPM authentication error

[chrisrbnelson]

What changed

• add XpmRuby::AuthenticationUnsuccessful as a subtype of XpmRuby::Forbidden
• raise it when a 403 response has the exact AuthenticationUnsuccessful detail
• keep every other 403 as the generic XpmRuby::Forbidden

Why

Consumers need to apply a dedicated retry and exhaustion policy to invalid XPM tenant authorisation without suppressing permission, edge, or other generic 403 failures.

Validation

• full RSpec suite: 93 examples, 0 failures
• Ruby syntax checks
• diff whitespace check

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