fix: preserve axios default Accept header when setting User-Agent (#7820)

Assigning `defaults.headers.common = { 'User-Agent': ... }` replaced the
entire common headers object, nuking axios's built-in default:

  Accept: application/json, text/plain, */*

This caused servers relying on content-negotiation to receive requests
with no Accept header. Fix by extending the existing object with a
property assignment instead.

Add regression tests for both electron and CLI axios instances verifying
that Accept is preserved and User-Agent is set correctly.

Co-authored-by: Pragadesh-45 <temporaryg7904@gmail.com>

packages/bruno-cli/src/utils/axios-instance.js

@@ -92,10 +92,11 @@ function makeAxiosInstance({
     headers: {}
   });
 
-  // Set User-Agent manually (using transformRequest to delete headers instead)
-  instance.defaults.headers.common = {
-    'User-Agent': `bruno-runtime/${CLI_VERSION}`
-  };
+  // Extend common headers with User-Agent rather than replacing the object.
+  // axios.create() preserves defaults.headers.common = { Accept: 'application/json, text/plain, */*' }.
+  // Assigning a new object (= { 'User-Agent': ... }) would nuke that default, causing servers that
+  // rely on content-negotiation to receive requests with no Accept header.
+  instance.defaults.headers.common['User-Agent'] = `bruno-runtime/${CLI_VERSION}`;
 
   instance.interceptors.request.use((config) => {
     config.headers['request-start-time'] = Date.now();