[upstream #862] librechat.ai#862: fileConfig.stt is read but missing from fileConfigSchema #421

fileConfig.stt is merged and read in packages/data-provider/src/file-config.ts:1268-1270 (dev a7e4de8), but fileConfigSchema (line 662) has no stt key. The schema is not strict, so the block passes through unvalidated and TFileConfig does not type it. Found while auditing the file config reference in LibreChat-AI/docs#862.

Fix: add an stt entry (with supportedMimeTypes) to fileConfigSchema.