feat: MCP space - group permissions and field-level sharing

[menglinghan]

• Added group-based access control — MCPs can be assigned to user groups with EDIT/READ_ONLY/PRIVATE levels. Visibility is filtered by group membership for all users including admins.
• Added field-level sharing — users can choose which config fields (server URL, auth token, headers, etc.) to share when publishing to the community.
• Various UI fixes — corrected publishing messages, removed version-related copy from MCP, separated withdraw/unpublish button text by status, and made community-downloaded MCPs editable.

[codecov]

Codecov Report

❌ Patch coverage is 89.93711% with 16 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
backend/services/remote_mcp_service.py	86.02%	3 Missing and 10 partials ⚠️
backend/services/mcp_management_service.py	92.00%	1 Missing and 1 partial ⚠️
backend/consts/model.py	88.88%	1 Missing ⚠️

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