Split runtime and backend files by concern

[timazed]

Summary

• split AgentRuntime into focused extensions for messaging, threads, skills, and memory
• split CodexResponsesBackend transport models and stream parsing into dedicated companion files
• split AgentRuntimeTests into message, persona/skill, and memory test files
• add section markers so the remaining entry files are easier to scan

Verification

• swift test
• xcodebuild -project DemoApp/AssistantRuntimeDemoApp.xcodeproj -scheme AssistantRuntimeDemoApp -destination 'generic/platform=iOS Simulator' CODE_SIGNING_ALLOWED=NO build