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fix: Return HTTP 500 for routes with invalid backend refs

Gateway API spec requires HTTP 500 when all backends for a routing rule
are invalid or nonexistent. The controller returned 404 via a
fixed-response rule, which is incorrect. 404 should only be used for
the listener default action (no matching rule).

Add InvalidBackendRefFixedResponseStatusCode constant (500) and use it
in rule_manager for the invalid-backend case, keeping the listener
default action at 404.