Skills: support DI catalog registration and request-scoped service access #1941

Summary

Track dependency-injection registration and request-scoped service access for custom Skills catalogs, deferred from #1856 by agreement during review.

Review discussion: #1856 (comment)

Current behavior

• WithSkills(IMcpSkillCatalog) accepts an existing catalog instance shared across requests; there is no generic or factory registration overload.
• McpSkillRequestContext exposes request and caller information, but not a service provider or McpServer.
• A database-backed catalog can create its own scope through IServiceScopeFactory, but that scope is independent of the existing request scope. A shared catalog must not capture scoped services directly.

The Skills guide in #1856 documents these constraints. Proper DI integration was explicitly left out of that PR.

Follow-up scope

• Design generic and/or factory-based catalog registration, including the supported catalog lifetimes.
• Separately determine how catalogs can access services from the existing request scope, including any Core plumbing needed for raw request handlers.
• Define scope ownership, disposal (including async disposal), and behavior across supported transports without introducing captive scoped dependencies.
• Add focused tests and documentation for the chosen registration and lifetime model, including a database-backed catalog example.

Creating a separate scope per catalog call remains a valid interim approach; it must not be described as sharing the MCP request scope. This issue tracks the design and implementation, not a requirement to expand the initial Skills PR.