fix(user-pool): enforce client operations within pool scope

[unvalley]

Summary

• validate UserPoolId before listing or deleting pool clients
• ensure DeleteUserPoolClient only deletes a client that belongs to the specified pool
• add regression tests for wrong-pool and missing-pool cases

Why

DeleteUserPoolClient accepted UserPoolId but did not enforce pool ownership, allowing cross-pool deletion when a valid ClientId from another pool was supplied.

Verification

• cargo fmt
• cargo clippy -- -D warnings
• cargo test

Supersedes #20 (same fix rebuilt from main).