Use correct error response for cancellations (#2939)

### Motivation

While working on #2938, I noticed that our request cancellation response was incorrect. The spec determines that requests that got cancelled by the client need to return an error using the code for request cancelled ([see here](https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#cancelRequest) and see `RequestCancelled` [here](https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#errorCodes)).

### Implementation

Started returning an error with the right code, rather than a response with a `nil` body.

I also started running cancel notifications directly in the main thread without pushing it to the queue, which was another mistake. If we put that notification in the queue, then we're guaranteed to only process them **after** we finish running requests, which is useless. We need the ability to process them as soon as we receive the notification, so that we have enough time to cancel the requests.

### Automated Tests

Added a test to verify this.

Author: vinistock

Diff for: lib/ruby_lsp/base_server.rb

@@ -91,7 +91,8 @@ def start
         # The following requests need to be executed in the main thread directly to avoid concurrency issues. Everything
         # else is pushed into the incoming queue
         case method
-        when "initialize", "initialized", "textDocument/didOpen", "textDocument/didClose", "textDocument/didChange"
+        when "initialize", "initialized", "textDocument/didOpen", "textDocument/didClose", "textDocument/didChange",
+             "$/cancelRequest"
           process_message(message)
         when "shutdown"
           send_log_message("Shutting down Ruby LSP...")
@@ -133,6 +134,12 @@ def pop_response
       @outgoing_queue.pop
     end
 
+    # This method is only intended to be used in tests! Pushes a message to the incoming queue directly
+    sig { params(message: T::Hash[Symbol, T.untyped]).void }
+    def push_message(message)
+      @incoming_queue << message
+    end
+
     sig { abstract.params(message: T::Hash[Symbol, T.untyped]).void }
     def process_message(message); end
 
@@ -154,7 +161,11 @@ def new_worker
           # Check if the request was cancelled before trying to process it
           @mutex.synchronize do
             if id && @cancelled_requests.include?(id)
-              send_message(Result.new(id: id, response: nil))
+              send_message(Error.new(
+                id: id,
+                code: Constant::ErrorCodes::REQUEST_CANCELLED,
+                message: "Request #{id} was cancelled",
+              ))
               @cancelled_requests.delete(id)
               next
             end

Diff for: lib/ruby_lsp/utils.rb

@@ -173,6 +173,9 @@ class Error
     sig { returns(String) }
     attr_reader :message
 
+    sig { returns(Integer) }
+    attr_reader :code
+
     sig { params(id: Integer, code: Integer, message: String, data: T.nilable(T::Hash[Symbol, T.untyped])).void }
     def initialize(id:, code:, message:, data: nil)
       @id = id

Diff for: test/server_test.rb

@@ -744,6 +744,51 @@ def test_requests_to_a_non_existing_position_return_error
     assert_match("Request textDocument/completion failed to find the target position.", error.message)
   end
 
+  def test_cancelling_requests_returns_expected_error_code
+    uri = URI("file:///foo.rb")
+
+    @server.process_message({
+      method: "textDocument/didOpen",
+      params: {
+        textDocument: {
+          uri: uri,
+          text: "class Foo\nend",
+          version: 1,
+          languageId: "ruby",
+        },
+      },
+    })
+
+    mutex = Mutex.new
+    mutex.lock
+
+    # Use a mutex to lock the request in the middle so that we can cancel it before it finishes
+    @server.stubs(:text_document_definition) do |_message|
+      mutex.synchronize { 123 }
+    end
+
+    thread = Thread.new do
+      @server.push_message({
+        id: 1,
+        method: "textDocument/definition",
+        params: {
+          textDocument: {
+            uri: uri,
+          },
+          position: { line: 0, character: 6 },
+        },
+      })
+    end
+
+    @server.process_message({ method: "$/cancelRequest", params: { id: 1 } })
+    mutex.unlock
+    thread.join
+
+    error = find_message(RubyLsp::Error)
+    assert_equal(RubyLsp::Constant::ErrorCodes::REQUEST_CANCELLED, error.code)
+    assert_equal("Request 1 was cancelled", error.message)
+  end
+
   private
 
   def with_uninstalled_rubocop(&block)