Reject unsupported LLM model identifiers with HTTP 400 on all analysis
endpoints (single, batch, async batch). The SUPPORTED_MODELS list was
already defined for the /models endpoint but not enforced on incoming
requests. This completes the multi-model support feature by adding the
missing server-side validation.
Closesleeworks-agents/SPARC#1013
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>