[FEATURE]: Implement MCP Apps ui:// resource storage and serving

[vishu-bh]

Parent epic: #2527

User Story

As a server administrator, I want ContextForge to store and serve MCP Apps UI resources through the generic extension governance layer so that apps can be represented as standard MCP ui:// resources without bypassing security policy.

One-Week Deliverable

Bring UI resource persistence and MCP resource-read serving under extension policy, RBAC/token scoping, plugin hooks, sanitization, and audit.

Acceptance Criteria

• UI resources are stored with ui:// URI, content metadata, MIME type, CSP config, permissions policy, owner/team visibility, source gateway, and audit fields.
• resources/read can return authorized ui:// resource content in MCP resource format only when MCP Apps is enabled and advertised through extension policy.
• Unauthorized, wrong-team, public-only, disabled-extension, and hidden-gateway callers cannot read private/team-inaccessible UI resources.
• Resource responses pass through configured plugin and sanitizer hooks before reaching the host.
• Invalid URI, unsafe MIME type, unsafe metadata, or disallowed external domain declarations are rejected or quarantined according to policy.
• Tests cover create/read/list, visibility scoping, invalid URI handling, disabled extension deny, and sanitizer/plugin denial.

Demo

Register ui://dashboard, read it through resources/read as an authorized caller, then show unauthorized and disabled-extension callers denied.