fix(Response): initialize #url field to prevent crash on Windows

The Response constructor and Response.error() were not initializing the
#url field, leaving it with uninitialized memory. When calculateEstimatedByteSize()
was called (which happens in toJS()), it would call byteSlice() on the
uninitialized String, causing a "switch on corrupt value" panic when the
String.tag field contained garbage.

This fix explicitly initializes #url to bun.String.empty in both:
- Response.constructor()
- Response.constructError()

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/bun.js/webcore/Response.zig

@@ -668,6 +668,7 @@ pub fn constructError(
             .#body = Body{
                 .value = .{ .Empty = {} },
             },
+            .#url = bun.String.empty,
         },
     );
 
@@ -751,6 +752,7 @@ pub fn constructor(globalThis: *jsc.JSGlobalObject, callframe: *jsc.CallFrame, j
     var response = bun.new(Response, Response{
         .#body = body,
         .#init = _init,
+        .#url = bun.String.empty,
         .#js_ref = .initWeak(js_this),
     });