refactor(admin): extract shared check helper

[JuanVP2]

Closes #289

Summary

Audited and refactored all 23 admin-gated entrypoints in src/lib.rs to consistently call Self::require_admin(&env); at the top of their execution path.

• Covered all entrypoints across governance, pair lifecycle, fee configuration, parameter setters, and role management.
• Preserved exact behavior, rejection conditions, and typed error codes (RouterError::NotInitialized, ContractPaused, etc.).
• Zero ABI breaking changes.

Verification Results

• cargo test: 247/247 unit and integration tests passed in green.
• cargo clippy --all-targets -- -D warnings: 0 warnings.
• cargo fmt --all -- --check: Formatting checked cleanly.