Fix https proxy not set when pac is used

[chemwolf6922]

Summary of the Pull Request

PAC is not directly usable in WSL or in most headless Linux environments.
The WinHttpGetProxySettingsEx call only populates the pcwszProxy segment when PAC is enabled. And WSL only sets http_proxy but leaves https_proxy empty.
This PR sets both http_proxy and https_proxy to the default PAC query result as best effort.

PR Checklist

• Closes: wslc pull (and WSL autoProxy) fail behind a PAC/Zscaler proxy: http_proxy is set but HTTPS_PROXY is left empty, so HTTPS registry pulls bypass the proxy and time out #40945
• 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

New unit tests:
NetworkTests::NetworkTests::NatHttpProxyPac
NetworkTests::MirroredTests::HttpProxyPac

[Copilot]

Pull request overview

This PR addresses a WSL autoProxy/PAC scenario where WinHTTP proxy settings derived from a PAC script populate only the generic proxy field, causing WSL to set http_proxy but leave https_proxy empty. The change makes WSL populate https_proxy/HTTPS_PROXY from the PAC-derived proxy value when no secure proxy is provided, and adds regression coverage for both NAT and mirrored networking modes.

Changes:

• Update environment injection to set https_proxy/HTTPS_PROXY to the PAC-derived proxy when SecureProxy is empty.
• Extend network tests to cover PAC-based proxy configuration in both NAT and mirrored networking.
• Ensure test cleanup clears AutoconfigUrl when resetting WinHTTP proxy settings.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
test/windows/NetworkTests.cpp	Adds NAT + mirrored tests for PAC-based WinHTTP proxy propagation and updates proxy-reset helper to clear AutoconfigUrl.
src/windows/service/exe/LxssUserSession.cpp	Falls back to setting https_proxy/HTTPS_PROXY from Proxy when PAC is configured and SecureProxy is empty.