v1SearchSchema.provider was a hard-coded z.enum that rejected any id outside
its list before the route's own resolveSearchProvider() check ever ran,
so unknown/short-alias provider ids (grok, brave, serper, ...) always
surfaced a generic "Invalid request" instead of the informative
"Unknown search provider: <id>" message. Relax the schema to a free-form
string and let resolveSearchProvider() own runtime validation (as it
already did for ids that passed the enum). Also extend
SEARCH_PROVIDER_ALIASES with short-form aliases mirroring the existing
jina/jina-ai pattern (brave, serper, perplexity, exa, tavily, google-pse,
linkup, ollama, searchapi, youcom, searxng, zai, duckduckgo), and surface
the first Zod validation issue's field name instead of the generic
message for other still-invalid fields (e.g. search_type).
