SI-6: Introduce ApiClient — a high-level typed client over protocol DTOs

[josecelano]

Subissue of EPIC #1938 — REST API Contract-First Migration

Problem

The REST API client package (torrust-tracker-rest-api-client) currently exposes only a low-level Client where all methods return raw reqwest::Response. Callers parse the body manually. Some methods panic via .unwrap(). External consumers (Torrust Index, E2E tests) build their own ad-hoc wrappers.

Decision

Introduce a two-tier client architecture in packages/rest-api-client/src/v1/client.rs:

• ApiHttpClient (renamed from Client) — low-level HTTP transport. Handles connection info, URL building, auth headers. Returns raw Response.
• ApiClient (new) — high-level typed client wrapping ApiHttpClient. Returns Result<DtoType, ClientError>. Never panics.

Scope

• Rename existing Client → ApiHttpClient
• Add rest-api-protocol as dependency
• Define ClientError enum (transport, deserialization, API error responses)
• Add ApiClient struct before ApiHttpClient in client.rs
• Implement typed methods on ApiClient for all endpoints

Checklist

• Client renamed to ApiHttpClient across codebase
• rest-api-protocol added as dependency
• ClientError enum defined
• ApiClient struct with typed methods for all endpoints added
• ApiClient appears before ApiHttpClient in client.rs
• All existing tests pass unchanged
• Pre-commit and pre-push checks pass