Handle missing specs when listing transitive dependencies (#3012)

### Motivation

This PR fixes a mistake made in #2937. We cannot invoke `to_spec` without rescuing `Gem::MissingSpecError` because that gets raised whenever there's a gem that doesn't match platform constraints.

### Implementation

Added the missing rescue.

### Automated Tests

Added a test that fails before the fix so that we catch this next time.

lib/ruby_indexer/lib/ruby_indexer/configuration.rb

@@ -273,7 +273,13 @@ def initial_excluded_gems
       end
 
       others.concat(this_gem.to_spec.dependencies) if this_gem
-      others.concat(others.filter_map { |d| d.to_spec&.dependencies }.flatten)
+      others.concat(
+        others.filter_map do |d|
+          d.to_spec&.dependencies
+        rescue Gem::MissingSpecError
+          nil
+        end.flatten,
+      )
       others.uniq!
       others.map!(&:name)
 

lib/ruby_indexer/test/configuration_test.rb

@@ -204,5 +204,36 @@ def test_transitive_dependencies_for_non_dev_gems_are_not_excluded
         end
       end
     end
+
+    def test_does_not_fail_if_there_are_missing_specs_due_to_platform_constraints
+      Dir.mktmpdir do |dir|
+        Dir.chdir(dir) do
+          File.write(File.join(dir, "Gemfile"), <<~RUBY)
+            source "https://rubygems.org"
+            gem "ruby-lsp", path: "#{Bundler.root}"
+
+            platforms :windows do
+              gem "tzinfo"
+              gem "tzinfo-data"
+            end
+          RUBY
+
+          Bundler.with_unbundled_env do
+            capture_subprocess_io { system("bundle install") }
+
+            _stdout, stderr = capture_subprocess_io do
+              script = [
+                "require \"ruby_lsp/internal\"",
+                "RubyIndexer::Configuration.new.indexable_uris",
+              ].join(";")
+
+              system("bundle exec ruby -e '#{script}'")
+            end
+
+            assert_empty(stderr)
+          end
+        end
+      end
+    end
   end
 end