Harden input validation for string fields (empty/whitespace/invalid forms)

[rawkode]

Problem

Some constructors validate inputs, but several request string fields can still be empty/whitespace/invalid (for example permission, relation, resource_type, subject_type).

Impact

Invalid request values can propagate deeper into execution and fail later with less actionable errors. This increases debugging cost and can produce inconsistent API behavior.

Proposed change

• Add consistent validation for required string fields across request builders/constructors.
• Reject empty, whitespace-only, and structurally invalid values.
• Fail fast with Error::InvalidArgument and field-specific error messages.
• Add test coverage for valid and invalid inputs across affected request types.

Acceptance criteria

• All required string request fields are validated consistently.
• Empty/whitespace/invalid forms return Error::InvalidArgument before request dispatch.
• Tests verify both rejection cases and valid pass-through behavior.