source: split CodexAppServer mapping helpers

[gaelic-ghost]

Summary

• keep CodexAppServer.swift focused on the actor/runtime implementation
• move approval and elicitation protocol response payload helpers into CodexAppServer+ProtocolPayloads.swift
• move generated-wire to public model mapping extensions into CodexAppServer+WireMapping.swift

Notes

• The moved helpers are internal instead of same-file private so CodexAppServer can keep using them across files; no public API is added.

Verification

• swift test
• swift build
• git diff --check
• bash scripts/repo-maintenance/validate-all.sh