Add `MongooseExtractor` for Mongoose schema definitions

[itsflaid]

Context
Same reserved slot in extractors/index.ts (// new MongooseExtractor()).

Scope
Parse new Schema({...}) / mongoose.Schema(...) definitions. Fields come from schema object keys; relations from ref: 'ModelName'.

Acceptance criteria

• canHandle() detects Mongoose usage (import from mongoose)
• extract() maps schema keys to FieldInfo, ref fields to isRelation
• Registered in EXTRACTORS, distinct from the raw-driver NoSQLExtractor in Document agent contribution workflow #1
• source union extended with "mongoose"
• Tests added