feat(handler): sanitize sensitive fields in form data

- Removes `username` and `apiKey` from form data to prevent errors
  from the LanguageTool server when these fields are present
- Updates test cases to reflect the new handling of form data bodies

src/__tests__/ltProxyHandler.test.ts

@@ -16,12 +16,16 @@ Deno.test('ltProxyHandler: proxies request and returns response', async () => {
     const originalFetch = globalThis.fetch;
     globalThis.fetch = async () => expectedResponse;
 
+    // Form body wie bei echtem Request
+    const formData = new URLSearchParams({ text: 'Hallo Welt' });
+    const bodyBytes = new TextEncoder().encode(formData.toString());
+
     const req = new Request('http://localhost/v2/check?language=de-DE', {
         method: 'POST',
-        body: new TextEncoder().encode('text=Hallo+Welt'),
         headers: {
             'content-type': 'application/x-www-form-urlencoded',
         },
+        body: bodyBytes,
     });
 
     const ctx: IContext = {
@@ -30,7 +34,9 @@ Deno.test('ltProxyHandler: proxies request and returns response', async () => {
         query: {
             language: 'de-DE',
         },
-        state: {},
+        state: {
+            body: bodyBytes,
+        },
     };
 
     const response = await ltProxyHandler(ctx);