separate `WebClient` and `MockWebClient` in the rust section

After merging 0xMiden/miden-client#1096, the access to the `MockRpcApi` is done via the functions in `crates/web-client/src/mock.rs` which belong to the `WebClient` struct. Ideally we would want these functions to only be accesible if the client uses a mocked RPC api.
This will require some refactoring, we should try to reduce code duplication, we wouldn't want all the logic to be copied twice.

> I wonder if this should be a fully separate class. I think this would allow for better separation (for example, the `serialize_mock_chain()` call would not be included for `WebClient`; and maybe we could have other helpers that are only available for the mocked client). Of course, this probably implies changing the service worker and some other parts f the JS side. If you agree, let's open an issue to track this.

_Originally posted by @igamigo in https://github.com/0xMiden/miden-client/pull/1096#discussion_r2271161967_

The issue is about refactoring the Rust web client to separate `WebClient` and `MockWebClient`. Start by examining `crates/web-client/src/mock.rs` and the `WebClient` struct to understand the current coupling. Look for traits or conditional compilation to reduce duplication. Changes will also affect the JavaScript side, including the service worker. 'Done' means the mock-specific functions are only accessible when using a mocked RPC API, without significant code duplication.

Escrito por el modelo de indexación a partir del texto del issue.