feat: add statuspage migration url name input

[debidong]

Summary

• Add optional URLName to StartStatusPageMigrationInput.
• Send url_name to /status-page/migrate-structure only when it is non-empty.
• Cover both omitted and provided url_name request bodies in SDK tests.

Test plan

• env -u GOROOT GOCACHE=/tmp/flashduty-sdk-gocache go test ./...
• git diff --check HEAD~1..HEAD

Note: the branch was pushed with --no-verify because the repo pre-push hook runs plain go test ./... and this machine has a Go stdlib/tool mismatch (go1.25.1 compiled stdlib vs go1.26.2 tool). The explicit clean-cache test above passed.