Use lsp_types directly in sway-lsp tests instead of simulating a client with tower-lsp

[JoshuaBatty]

Description

This PR removes the mock client <> server interaction to test the language server features in favour of using lsp_types directly. There are a few places like did_change where we are still using the exisiting framework, this is because we still rely on the tower-lsp event loop to synchronise notification messages. This will be tackled with the custom event loop work. Should be easy to just update the remaining tests here within that PR. I've also kept the publish diagnostics test the same as we still rely on the Client from tower-lsp for that functionality.

There were also a few tests here that were unnecessary. Namely, initialized initializes_only_once shutdown refuses_requests_after_shutdown. These tests are actually duplicated from tower-lsp itself, so there is really no reason we need to run them again in sway-lsp.

working towards #4958

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[sdankel]

Nice to have this decoupling! LGTM