[Bug][Notion] Apply backend OAuth fixes (parity with Gmail OAuth fix)

[Al629176]

Summary

The OAuth fix applied on the backend for Gmail must be applied to Notion as well so Notion skill connect flows behave correctly (redirect URIs, client config, scopes, and error handling).

Context

• Gmail OAuth was corrected on the backend.
• Notion integration still uses the old path or is missing the same fixes.

Goal

• Notion OAuth/connect should match the corrected Gmail pattern: same reliability, validation, and user-visible outcomes.

Tasks

• Audit Gmail backend OAuth changes (redirect URIs, env, token exchange, error mapping).
• Align Notion OAuth with the same patterns (no duplicate or stale callback URLs).
• Verify Google Cloud / Notion app settings match deployed callbacks.
• Add or update integration tests for Notion connect + token refresh.
• Document any env vars or deployment steps for Notion parity.

Acceptance criteria

• Notion skill OAuth completes successfully in the same environments where Gmail works.
• No "wrong redirect" / stale client / generic failure for known-good configs.
• Errors are actionable and consistent with Gmail messaging where applicable.