[breaking] DeepSeek deprecates the deepseek-chat and deepseek-reasoner model names, remapping them to DeepSeek-V4-Flash modes

The deepseek-chat and deepseek-reasoner model names are deprecated effective July 24, 2026 at 15:59 UTC. For compatibility the slugs continue to work, remapped to the non-thinking and thinking modes of deepseek-v4-flash respectively — a stable-ID swap to a different underlying model. The pricing page does not date the announcement; announced_at reflects when the notice was first verified into this catalog (2026-06-30). Effective: 2026-07-24. Migration target: deepseek/deepseek-v4-flash. Affected: deepseek/deepseek-chat, deepseek/deepseek-reasoner. Severity: breaking. Verification: verified.