API Builder V1: Docs guard + publish flow polish

[nicdavidson]

Goal
Close build-to-docs loop reliably.

Scope

• Open Docs only after API saved and base_path exists
• Pre-check /api_docs/<base_path> before route navigation
• Show explicit toast on failure path

Acceptance Criteria

• No blind redirects to broken docs pages
• User gets reason + next fix step when docs unavailable

[nicdavidson]

Delivered as part of API Builder V1 (adf-api-builder module). Closing tracking issue.