test: extend registered_clients admin gate tests with DB state checks

[sergey3bv]

Summary

• Extend registered_clients_admin_http_test so forbidden paths seed a real
  row, then assert the row is unchanged.

Avoid naming corporate partners, customers, or other sensitive external brands in this PR title or body. Use generic descriptors unless a maintainer explicitly approves the public reference.

Motivation

• A follow-up for test(admin): add negative HTTP-layer tests for registered_clients full-admin gate #175

Related Issue

• Closes Follow-up: add DB-state assertions for rejected registered_clients update/delete paths #183

Testing

• cargo test --workspace --verbose
• cargo clippy --workspace --all-targets --all-features -- -D warnings -A deprecated
• cargo fmt --all -- --check
• Manual verification completed

Visuals

• UI/web change with screenshots/video attached
• No visual change
• Visuals and text avoid sensitive external brand or partner names unless explicitly approved