Fix incorrect string -> wstring conversion, causing utf16 corruption

[Blue (OneBlue)]

Summary of the Pull Request

The issue was using std::format_to with a narrow string, which would actually convert the string back to utf8, and writing utf8 bytes on a wide string, causing corruption

PR Checklist

• Closes: Link to issue #xxx
• Communication: I've discussed this with core contributors already. If work hasn't been agreed, this work might be rejected
• Tests: Added/updated if needed and all pass
• Localization: All end user facing strings can be localized
• Dev docs: Added/updated if needed
• Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Validation Steps Performed

[Copilot]

Pull request overview

This PR fixes UTF-16 corruption when formatting UTF-8 char strings into wide (wchar_t) formatted output by ensuring the wide-formatting formatter specializations use wide format strings.

Changes:

• Fix std::formatter<... , wchar_t> specializations in stringshared.h to use L"{}" (wide) instead of "{}" (narrow) when writing to a wide format context.
• Add a Windows unit test validating std::format(L"{}", std::string) produces the expected wide string for UTF-8 input.
• Add a WSLC E2E test that exercises Unicode build output and asserts the expected Unicode substring is present.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
test/windows/wslc/e2e/WSLCE2EImageBuildTests.cpp	Adds an E2E test that validates Unicode output from wslc build is not corrupted.
test/windows/StringUnitTests.cpp	Adds a unit test covering UTF-8 std::string → wide std::format behavior.
src/shared/inc/stringshared.h	Fixes wide-context std::formatter implementations to format using wide format strings, preventing UTF-16 corruption.

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