Improve Notion OAuth HTTPS error guidance

[Pandakingxbc]

What changed

• Detect a public HTTP callback before starting Notion OAuth.
• Return a dedicated CONNECTOR_OAUTH_HTTPS_REQUIRED error with actionable localized guidance instead of the generic invalid-credentials message.
• Keep the check scoped to Notion; loopback HTTP and all other connectors retain their existing behavior.

Root cause

The OAuth callback is derived from request.base_url. When Octop is accessed through a public HTTP address, Notion rejects dynamic client registration because non-loopback Notion callback URLs must use HTTPS. The existing error path then presents the generic connector credentials message.

Impact

Only Notion OAuth on a public HTTP address receives the new error. No Docker, configuration, callback construction, or MCP transport behavior is changed.

Validation

• make lint
• Focused pytest suite: 21 passed
• The integration test verifies that Notion OAuth is not started for a public HTTP callback and that the dedicated error code is returned