Fix OAuth authorization resume after sign-up

[AliOsm]

Summary

• resume the saved OAuth authorization URL after a user creates an account
• retain the existing dashboard destination for ordinary signups with no saved return path
• add an integration test covering authorize request, signup, exact consent URL restoration, and consent rendering
• update the Doorkeeper authentication comment to describe both sign-in and sign-up resume behavior

Root cause

start_new_session_for correctly preserved session[:return_to_after_authenticating] across session rotation, but UsersController#create always redirected successful registrations to the dashboard. Unlike sign-in, signup never consumed the saved OAuth URL, so a new user connecting an MCP client left the client waiting for an authorization callback.

User impact

A first-time PasteHTML user can now choose Sign up during a Claude, Codex, or other MCP OAuth flow and return directly to the consent screen. Existing signup behavior remains unchanged outside an interrupted authentication flow.

Validation

• focused OAuth and authentication suites: 33 runs, 228 assertions, 0 failures, 0 errors
• CI=1 bundle exec rails test: 440 runs, 2,053 assertions, 0 failures, 0 errors
• bundle exec rubocop: 128 files, no offenses
• bundle exec brakeman --no-pager: zero warnings
• RAILS_ENV=test bundle exec rails zeitwerk:check: all good