Add @oagen-ignore-start/end to hand-written methods in mixed files

Protects hand-written methods from being overwritten during oagen
regeneration while allowing the merger to add new generated methods
alongside them.

Files with ignore regions:
- user-management.ts: session helpers, PKCE, MFA wrappers, URL builders
- sso.ts: getAuthorizationUrl, getAuthorizationUrlWithPKCE
- feature-flags.ts: createRuntimeClient
- widgets.ts: getToken

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>