Refactor SaveImageImpl and ExportContainerImpl to eliminate code duplication

[Copilot]

Summary of the Pull Request

Addresses code review feedback from #14121 requesting extraction of ~80% duplicated code between SaveImageImpl and ExportContainerImpl into a shared helper function.

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

Extracted common logic into HandleImageLikeOperation helper

• Consolidates HTTP response handling, data relay setup, and error parsing
• Takes optional callback parameter for operation-specific logic (e.g., WSL_LOG in export)
• Removes redundant m_dockerClient validation already performed by callers

Before:

void WSLASession::SaveImageImpl(...) {
    wil::unique_handle imageFileHandle{...};
    THROW_HR_IF(HRESULT_FROM_WIN32(ERROR_INVALID_STATE), !m_dockerClient.has_value());
    relay::MultiHandleWait io;
    auto onCompleted = [&]() { io.Cancel(); };
    std::string errorJson;
    auto accumulateError = [&](const gsl::span<char>& buffer) { ... };
    // ... 30+ lines of identical logic
}

void WSLASession::ExportContainerImpl(...) {
    // Nearly identical code with minor variations
}

After:

void HandleImageLikeOperation(
    std::pair<uint32_t, std::unique_ptr<DockerHTTPClient::HTTPRequestContext>>& RequestCodePair,
    ULONG OutputHandle,
    HANDLE SessionTerminatingEvent,
    WSLA_ERROR_INFO* ErrorInfo,
    const char* FailureMessageFormat,
    std::function<void()> OnCompletedCallback = nullptr)
{
    // Consolidated logic with callback extension point
}

void WSLASession::SaveImageImpl(...) {
    HandleImageLikeOperation(RequestCodePair, OutputHandle, m_sessionTerminatingEvent.get(), 
                            Error, "Image save failed: %hs");
}

void WSLASession::ExportContainerImpl(...) {
    auto onCompletedCallback = []() { WSL_LOG("OnCompletedCalledForExport", ...); };
    HandleImageLikeOperation(RequestCodePair, OutputHandle, m_sessionTerminatingEvent.get(),
                            Error, "Container export failed: %hs", onCompletedCallback);
}

Net change: -1 LOC (67 removed, 66 added) with improved maintainability.

Validation Steps Performed

This PR targets the base branch of #14121 (not main). Validation will occur when #14121 is built and tested on Windows with Visual Studio.

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

[Copilot]

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• archives.boost.io
  • Triggering command: /usr/local/bin/cmake /usr/local/bin/cmake -DCMAKE_MESSAGE_LOG_LEVEL=VERBOSE -P /home/REDACTED/work/WSL/WSL/_codeql_build_dir/_deps/x64/boost_headers-subbuild/boost_headers-populate-prefix/src/boost_headers-populate-stamp/download-boost_headers-populate.cmake (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)