[cr143] Extension::Create taking a u16string arg

This used to be a `std::string`.

Chromium changes:
https://chromium.googlesource.com/chromium/src/+/79167e224d3cd322af1031bb0124212580e7cde8

commit 79167e224d3cd322af1031bb0124212580e7cde8
Author: Jaewon Jung <jw.jung@navercorp.com>
Date:   Thu Oct 2 19:04:02 2025 -0700

    Use std::u16string for Extension::Create() error parameter

    The `Extension::Create()` method and its callers used `std::string` for
    the error output parameter, leading to unnecessary conversions between
    UTF-8 and UTF-16.

    This change migrates the error parameter to `std::u16string` across
    the call stack. This eliminates string conversions, simplifying the
    code and improving consistency. In a few places where a `std::string`
    is still required, a temporary conversion is performed.

    Signed-off-by: Jaewon Jung <jw.jung@navercorp.com>
    Bug: 41317803
    Change-Id: Ia660ca60834f233eca82e8f2b65961cce48230d6
    Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6975452
    Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
    Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
    Cr-Commit-Position: refs/heads/main@{#1524554}

browser/brave_wallet/external_wallets_importer.cc

@@ -226,7 +226,7 @@ void ExternalWalletsImporter::GetLocalStorage(
     const extensions::Extension& extension,
     InitCallback callback) {
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
-  std::string error;
+  std::u16string error;
   extension_ = Extension::Create(
       extension.path(), ManifestLocation::kExternalPref,
       *extension.manifest()->value(), extension.creation_flags(), &error);

browser/extensions/mv2_warning_unittest.cc

@@ -32,7 +32,7 @@ TEST(Mv2WarningTest, ExtensionManifestVersions) {
     return dict;
   };
 
-  std::string error;
+  std::u16string error;
   scoped_refptr<extensions::Extension> extension =
       extensions::Extension::Create(
           base::FilePath(), extensions::mojom::ManifestLocation::kUnpacked,