Add missing synchronization around load WinHttp proxy methods

[Blue (OneBlue)]

Summary of the Pull Request

This change moves the entire loading of the winhttp methods under the std::call_once, otherwise two threads could be trying to load the methods in parallel and write the global std::optionals at the same time

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]

🟡 Changes recommended

Failed function loads are incorrectly converted to success, leaving required optionals empty.

1 open finding

• Preserve WinHTTP loader failure HRESULT instead of returning S_OK · New

What changed in this PR

Moves WinHTTP API loading under one-time synchronization to prevent concurrent writes.

Changes:

• Synchronizes module and function loading with std::call_once.
• Caches the load result for subsequent callers.

File	Description
src/​windows/​service/​exe/​LxssHttpProxy.cpp	Makes WinHTTP proxy API initialization thread-safe.

🧠 Review effort: Balanced

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[Copilot]

🟢 Approval recommended

The focused change correctly serializes initialization and safely publishes its result to concurrent callers.

0 open findings

1 resolved since last review

• Preserve WinHTTP loader failure HRESULT instead of returning S_OK

🧠 Review effort: Balanced