Add auth recovery, request retries, and reasoning effort UI

[timazed]

Summary

• add CLI-style unauthorized session recovery and transient request retry/backoff to the runtime/backend
• add configurable ReasoningEffort support and send reasoning.effort to the Responses API
• add demo app UI to switch thinking level for future requests and document the new configuration in the README

Testing

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