fix(namegen): retry transient upstream failures in provisioning

[aa-parky]

Summary

• add bounded retry semantics to character provisioning namegen lookup for transient failures
• retry request exceptions and retryable HTTP statuses (408/429/5xx)
• keep non-retryable HTTP and payload validation failures fail-fast
• expand unit tests to verify retry behavior and no-retry behavior explicitly

Testing

• PYENV_VERSION=pms pytest -q tests/test_api/test_admin_create_character.py
• PRE_COMMIT_HOME=/tmp/pre-commit-cache PYENV_VERSION=pms pre-commit run --files src/mud_server/services/character_provisioning.py tests/test_api/test_admin_create_character.py