fix: Return type, make config non optional

Co-authored-by: Alois Klink <alois@aloisklink.com>

packages/mermaid/src/preprocess.ts

@@ -49,7 +49,7 @@ const processDirectives = (code: string) => {
  * @param code - The code to preprocess.
  * @returns The object containing the preprocessed code, title, and configuration.
  */
-export function preprocessDiagram(code: string): DiagramMetadata & { code: string } {
+export function preprocessDiagram(code: string) {
   const cleanedCode = cleanupText(code);
   const frontMatterResult = processFrontmatter(cleanedCode);
   const directiveResult = processDirectives(frontMatterResult.text);
@@ -59,5 +59,5 @@ export function preprocessDiagram(code: string): DiagramMetadata & { code: strin
     code,
     title: frontMatterResult.title,
     config,
-  };
+  } satisfies DiagramMetadata & { code: string };
 }

packages/mermaid/src/types.ts

@@ -53,7 +53,7 @@ export interface ParseResult {
   /**
    * The config passed as YAML frontmatter or directives
    */
-  config?: MermaidConfig;
+  config: MermaidConfig;
 }
 // This makes it clear that we're working with a d3 selected element of some kind, even though it's hard to specify the exact type.
 export type D3Element = any;