docs(auth): update docs for new invocation flow

[masnwilliams]

Note

Modernizes Agent Auth docs to the new invocation model and credentials behavior.

• Switches examples to a polling-based state machine (status, step) with pending_fields/submitted_fields; removes ALREADY_AUTHENTICATED branching and treats submit() as async
• Adds explicit FAILED handling (with error_message) and terminal state checks; updates tables and polling snippets accordingly
• Replaces reauth() with creating a new invocations.create and polling for completion across Auto-Login, Hosted UI, and Session Monitoring
• Standardizes credential linking to credential_name (not credential_id); documents save_credential_as merging into existing credentials
• Programmatic flow reworked from discover/submit to poll→submit cycles with TOTP/code support and UI examples
• Minor doc tweaks: improved examples, proxy usage consistency, and notes/warnings on 2FA codes not being saved

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