Function extractValidators

Walk a defineAPI() result and collect ~validators metadata from defineMethod() handlers into an RPCValidators-shaped object.

For each key in the API object:

• If it's a function with ~validators metadata → extract the schemas
• If it's a plain object (namespace) → recurse into it
• If it's a Standard Schema object (has ~standard) → skip it (this prevents treating schema objects as namespace nodes)
• Plain functions without metadata are skipped (no validation for them)

The result can be passed directly to RPCChannel's validators option.