refactor(catalog/live): split fetchers.go by provider group

Move OpenAI/OpenRouter enrichment and cloud-provider (Azure, Bedrock,
Vertex, MiniMax) fetchers into fetchers_cloud.go, and the remaining
per-provider fetchers into fetchers_providers.go. The registry, shared
parsing/pricing/env helpers, and AWS SigV4 signing helpers remain in
fetchers.go. Pure mechanical split, no behavior changes.

Author: Patel230