The Anthropic model claude-sonnet-4-0 returns HTTP 404 for current API
keys; it is no longer a reachable alias. Remove it from AIMetadata so the
model picker and the per-model live harness sweeps stop offering it.

Also delete the now-orphaned captured fixtures that only existed for this
model: its two SafetyRefusalFixtures and the 16 AICassettes whose request
body targets claude-sonnet-4-0. The live harness derives its model list
from AIMetadata, so there are no per-model test methods to remove; the
AIMetadataFixtureCoverageTest still passes because no remaining model
lacks a refusal fixture.