Feat: Support multipart/mixed (#7155)

* feat(): support multipart mixed

fix: support vars interpolation on mixed multi-part

Update packages/bruno-electron/src/ipc/network/interpolate-vars.js

Co-authored-by: Timon <39559178+Its-treason@users.noreply.github.com>

refactor: use startsWith

feat: best effort for other multipart/* contentypes

* feat: enhance variable interpolation for multipart requests

- Updated `interpolateVars` function to support interpolation in multipart/form-data and multipart/mixed requests.
- Added handling for empty multipart arrays and parts with missing or undefined values.
- Improved type checks for content types to ensure proper interpolation behavior.

Includes new tests to validate the interpolation functionality for multipart requests.

* fix: normalize error handling in sendRequest and improve test reliability

---------

Co-authored-by: Alfonso Presa <alfonso-presa@users.noreply.github.com>

packages/bruno-tests/collection/scripting/api/bru/cookies/getCookie.bru

@@ -18,8 +18,9 @@ script:pre-request {
 
 tests {
   const jar = bru.cookies.jar()
-  
-  jar.getCookie("https://testbench-sanity.usebruno.com", "name", function(error, data) {
+  // Await so the callback runs before jar.clear() below; otherwise the test script can finish
+  // before the callback registers/runs the test, causing a flaky failure (e.g. in CI).
+  await jar.getCookie("https://testbench-sanity.usebruno.com", "name", function(error, data) {
     if(error) {
       console.error("Cookie retrieval error:", error)
       throw new Error(`Failed to get cookie: ${error.message || error}`)