Implement hover menu support for ruff-server; Issue #10595 (#11096)

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## Summary

Add support for hover menu to ruff_server, as requested in
[10595](https://github.com/astral-sh/ruff/issues/10595).
Majority of new code is in hover.rs.
I reused the regex from ruff-lsp's implementation. Also reused the
format_rule_text function from ruff/src/commands/rule.rs
Added capability registration in server.rs, and added the handler to
api.rs.

## Test Plan

Tested in NVIM v0.10.0-dev-2582+g2a8cef6bd, configured with lspconfig
using the default options (other than cmd pointing to my test build,
with options "server" and "--preview"). OS: Ubuntu 24.04, kernel
6.8.0-22.

---------

Co-authored-by: Jane Lewis <me@jane.engineering>

crates/ruff_server/src/server.rs

@@ -257,6 +257,7 @@ impl Server {
                     },
                 },
             )),
+            hover_provider: Some(types::HoverProviderCapability::Simple(true)),
             text_document_sync: Some(TextDocumentSyncCapability::Options(
                 TextDocumentSyncOptions {
                     open_close: Some(true),