Validate custom install locations before downloading distributions

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Summary of the Pull Request

wsl --install --location deferred path creation until after downloading the distribution. Custom locations are now created and validated before payload acquisition, failing immediately when unusable.

PR Checklist

• 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

• Pre-creates custom install directories before distribution download.
• Removes newly created empty directories when installation fails.
• Preserves existing directories and service-side path conflict validation.

Validation Steps Performed

• Added regression coverage verifying an invalid location fails before network download begins.

• Fixes wsl --install --location downloads distribution before validating target directory existence #41625

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🟡 Changes recommended

Unresolved directory cleanup and path validation issues remain, along with a localization gap and missing regression coverage.

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Review details

Suppressed comments (3)

src/windows/common/WslInstall.cpp:294

• Failed to create install location: %ls is a newly introduced English string on the wsl --install error path, but it has no localization resource. Use a Message... helper with the path/error details as placeholders and THROW_HR_WITH_USER_ERROR so failures from this preflight validation follow the localized error handling used elsewhere in this component.

        THROW_IF_WIN32_ERROR_MSG(error.value(), "Failed to create install location: %ls", location->c_str());

src/windows/common/WslInstall.cpp:294

• create_directories treats an already-existing path as successful with no error even when it is a regular file. Because directoryCreated is false in that case, this proceeds to download and the service only fails later when it tries to install beneath the file, so the preflight still accepts an unusable --location. Check is_directory (and propagate its error) after this call before starting acquisition.

        directoryCreated = std::filesystem::create_directories(*location, error);
        THROW_IF_WIN32_ERROR_MSG(error.value(), "Failed to create install location: %ls", location->c_str());

test/windows/UnitTests.cpp:5683

• This test only exercises the error path where create_directories fails immediately because g_testDistroPath is a file. It does not cover the new directoryCreated == true branch (successful creation followed by download/import failure), so it would not catch the incomplete cleanup cases above. Add a nested, otherwise-valid location and assert that the newly created directories are removed after failure.

                auto [output, error] = LxsstuLaunchWslAndCaptureOutput(
                    std::format(L"--install invalid-location --no-launch --location \"{}\"", invalidLocation.native()), -1);
                VERIFY_IS_TRUE(output.find(L"Downloading:") == std::wstring::npos);
                VERIFY_IS_FALSE(error.empty());

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Lite