fix(api): enforce admin and RBAC permissions on tool OAuth custom client GET endpoint (#40944)

[agarwalpranav0711]

Summary

Fixes #40944

The GET /console/api/workspaces/current/tool-provider/builtin/<provider>/oauth/custom-client endpoint in ToolOAuthCustomClient was missing authorization decorators:

• @is_admin_or_owner_required
• @rbac_permission_required(RBACResourceScope.WORKSPACE, RBACPermission.CREDENTIAL_MANAGE, resource_required=False)

This allowed workspace members without admin/owner roles or credential management RBAC permissions to fetch custom OAuth client configuration details.

Added missing permission decorators to match sibling post and delete methods and added corresponding unit test assertion.

Screenshots

N/A (Backend API permission enforcement)

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint && make type-check (backend) and vp staged (frontend) to appease the lint gods

[github-actions]

Pyrefly Type Coverage

Metric	Base	PR	Delta
Type coverage	59.72%	59.72%	-0.00%
Strict coverage	59.30%	59.30%	-0.00%
Typed symbols	39,771	39,771	0
Untyped symbols	27,010	27,011	+1
Modules	3175	3175	0

[crazywoola]

Please update to the latest origin/main and fix the CI errors.