feat(ruby): add Fuzzy Ruby Server language server support

[candidosales]

Summary

Adds Fuzzy Ruby Server as a supported language server in the Ruby extension. This was requested by the Zed team after closing zed-industries/extensions#5107 — all Ruby language servers should live in this single extension rather than in separate extensions.

Fuzzy Ruby Server is a Rust binary (not a Ruby gem), so its integration intentionally bypasses the bundler/gemset installation logic used by the other language servers.

What's included

• Registers fuzzy-ruby-server in extension.toml
• Adds FuzzyRubyServer struct implementing the LanguageServer trait with a custom language_server_binary that:
  • Honors user-configured binary path via lsp.fuzzy-ruby-server.binary.path in Zed settings (with existence validation and a clear error message if the path is invalid)
  • Falls back to PATH lookup for the fuzzy binary
  • Returns a descriptive install hint if the binary is not found
• Extends WorktreeLike and FakeWorktree with a which() method for testability
• Adds 6 unit tests covering all resolution branches (custom path, missing path, PATH lookup, not found)

Installation

Users must install the binary manually:

cargo install --git https://github.com/doompling/fuzzy_ruby_server

Or configure an explicit path in Zed settings:

{
  "lsp": {
    "fuzzy-ruby-server": {
      "binary": {
        "path": "/path/to/fuzzy"
      }
    }
  }
}

Test plan

• All 57 unit tests pass (cargo test)
• Unit tests cover: valid custom path, invalid custom path, PATH lookup success, PATH lookup failure
• Manual: open a Ruby project in Zed with fuzzy installed and verify completions work
• Manual: configure an explicit binary path and verify it is used
• Manual: verify the error message when fuzzy is not installed