fix: preserve user-defined boundary in multipart/mixed Content-Type header (#7531)

* fix: preserve user-defined boundary in multipart/mixed Content-Type header

When users specify a boundary parameter in their Content-Type header for
multipart/mixed requests with TEXT body mode, Bruno now preserves the
user-defined boundary instead of generating a new one.

Fixes: https://github.com/usebruno/bruno/issues/7523

* updated the test to use local server and changed the request method to GET

* fix: handle quoted boundary values in Content-Type header extraction

---------

Co-authored-by: Chirag Chandrashekhar <cchirag85@gmail.com>

packages/bruno-tests/src/echo/index.js

@@ -5,6 +5,15 @@ router.get('/path/*', (req, res) => {
   return res.json({ url: req.url });
 });
 
+// Echo back request headers - useful for testing header manipulation
+router.all('/headers', (req, res) => {
+  return res.json({
+    method: req.method,
+    headers: req.headers,
+    url: req.url
+  });
+});
+
 router.post('/json', (req, res) => {
   return res.json(req.body);
 });