Check if lockfile is valid before composing bundle (#3077)

### Motivation

After #3066 moves forward, we can start getting smarter about our checks of when it's valid to restart. For example, we don't need to go all the way to composing the bundle, if we already know the lockfile's syntax is invalid.

### Implementation

Started parsing the lockfile before composing the bundle, so that we can more quickly reject a restart request if there's an issue.

### Automated Tests

Added a test.

lib/ruby_lsp/server.rb

@@ -1292,6 +1292,13 @@ def compose_bundle(message)
       command = "#{Gem.ruby} #{File.expand_path("../../exe/ruby-lsp-launcher", __dir__)} #{@global_state.workspace_uri}"
       id = message[:id]
 
+      begin
+        Bundler::LockfileParser.new(Bundler.default_lockfile.read)
+      rescue Bundler::LockfileError => e
+        send_message(Error.new(id: id, code: BUNDLE_COMPOSE_FAILED_CODE, message: e.message))
+        return
+      end
+
       # We compose the bundle in a thread so that the LSP continues to work while we're checking for its validity. Once
       # we return the response back to the editor, then the restart is triggered
       Thread.new do

test/server_test.rb

@@ -1056,6 +1056,55 @@ def test_compose_bundle_creates_file_to_skip_next_compose
     end
   end
 
+  def test_compose_bundle_detects_syntax_errors_in_lockfile
+    Dir.mktmpdir do |dir|
+      Dir.chdir(dir) do
+        @server.process_message({
+          id: 1,
+          method: "initialize",
+          params: {
+            initializationOptions: {},
+            capabilities: { general: { positionEncodings: ["utf-8"] } },
+            workspaceFolders: [{ uri: URI::Generic.from_path(path: dir).to_s }],
+          },
+        })
+
+        File.write(File.join(dir, "Gemfile"), <<~GEMFILE)
+          source "https://rubygems.org"
+          gem "stringio"
+        GEMFILE
+
+        # Write a lockfile that has a git conflict marker
+        lockfile_contents = <<~LOCKFILE
+          GEM
+            remote: https://rubygems.org/
+            specs:
+              <<<<<<< HEAD
+              stringio (3.1.0)
+              >>>>>> 12345
+
+          PLATFORMS
+            arm64-darwin-23
+            ruby
+
+          DEPENDENCIES
+            stringio
+
+          BUNDLED WITH
+            2.5.7
+        LOCKFILE
+        File.write(File.join(dir, "Gemfile.lock"), lockfile_contents)
+
+        Bundler.with_unbundled_env do
+          @server.process_message({ id: 2, method: "rubyLsp/composeBundle" })
+        end
+
+        error = find_message(RubyLsp::Error)
+        assert_match("Your Gemfile.lock contains merge conflicts.", error.message)
+      end
+    end
+  end
+
   private
 
   def with_uninstalled_rubocop(&block)