fix: respect merged UDP option in Clash export

[slackworker]

Summary

• Use the merged udp option when exporting Clash proxies instead of the raw node field.
• Ensure request/global udp=true|false overrides are reflected in final Clash output.
• Keep Snell behavior unchanged by continuing to omit the UDP field for Snell nodes.

Why

The previous implementation wrote x.UDP directly in the final Clash node output, which bypassed the already-merged effective udp value and prevented expected request-level override behavior.

Test Plan

• Reproduced the issue with a running local subconverter service.
• Verified behavior around Clash output generation path after the fix.
• Build was not completed locally due missing dependencies (CURL for full CMake build and yaml-cpp for static build).

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