Use std::variant for VHD device path storage instead of std::pair with optionals

[Copilot]

Summary of the Pull Request

Refactors VHD boot disk device path storage from std::pair<std::optional<ULONG>, std::optional<std::string>> to std::variant<ULONG, std::string> for improved type safety and clarity.

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

The rootVhd and modulesVhd variables store either a SCSI LUN (ULONG) or PMEM device path (string), but never both. The previous implementation used std::pair<std::optional<ULONG>, std::optional<std::string>> with runtime assertions to enforce mutual exclusivity.

Changes:

• Replace pair type with std::variant<ULONG, std::string>
• Remove runtime assertion - type system now guarantees correctness
• Implement visitor pattern with explicit overloads for device path resolution:

auto getVhdDevicePath = [&](const std::variant<ULONG, std::string>& vhd) {
    struct Visitor {
        WSLAVirtualMachine* vm;
        
        std::string operator()(ULONG lun) const {
            return vm->GetVhdDevicePath(lun);
        }
        
        std::string operator()(const std::string& path) const {
            return path;
        }
    };
    
    return std::visit(Visitor{this}, vhd);
};

Validation Steps Performed

Code formatted with clang-format and reviewed for correctness. Full Windows build validation deferred to CI.

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