Improve Candidate Validation

Schema-backed request validation is now used on candidate endpoints, returning clearer invalid_input messages when required fields are missing or malformed. Applies to the following endpoints:

• candidate.addEmailMessage
• candidate.addProject
• candidate.addTag
• candidate.anonymize
• candidate.create
• candidate.createNote
• candidate.info
• candidate.listClientInfo
• candidate.listFraudChecks
• candidate.listNotes
• candidate.listProjects
• candidate.removeTag
• candidate.search
• candidate.setFraudStatus
• candidate.update
• candidate.uploadFile
• candidate.uploadResume