Rewrite User & Group Management Client based on Pydantic Models

## Description

Rewrite user and group management client to use Pydantic models from the API.

## Tasks

- [ ] Generate client from Pydantic API models
- [ ] Implement type-safe user/group methods
- [ ] Add client-side validation
- [ ] Update client tests
- [ ] Update client documentation

## Acceptance Criteria

- Type-safe client interface
- Full IDE auto-completion
- All tests pass

## Dependencies

- Blocked by: BA-2633 (Migrate User & Group Management API to Pydantic)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

JIRA Issue: BA-2643

Start by reviewing the Pydantic API-model migration tracked as BA-2633, then inspect the existing user and group management client and its client tests. Done means the client uses the generated models, provides type-safe methods and validation, and has updated tests and documentation with all tests passing.

Written by the indexing model from the issue text.