mcp: add ListResourcesHandler for dynamic resources/list

[piyushbag]

Problem

resources/list is backed only by a static in-memory set populated via AddResource. Gateway servers that proxy upstream catalogs and template-served skill assets cannot enumerate resources on demand: reads work via ResourceHandler, but listing does not.

Approach

Add ListResourcesHandler on ServerOptions, analogous to ResourceHandler for reads:

• Handler only (no static resources): client pagination cursors pass through unchanged. Suitable for gateway/proxy servers.
• Handler + static resources: SDK paginates registered resources first, then invokes the handler for subsequent pages using an internal composite cursor.
• Capability: advertise resources when the handler is set, even if no resources were added statically.

resources/directory/read backing is intentionally deferred until custom-method support (#956) lands.

Why this helps

• First-class, typed registration instead of intercepting resources/list in middleware.
• Preserves existing static AddResource behavior when no handler is configured.
• Composes static and dynamic catalogs for template-backed servers that also register fixed entries.

Test plan

• go test ./mcp/ -run TestListResourcesHandler
• go test ./...
• go test -race ./mcp/ -run TestListResourcesHandler

Fixes #998

[piyushbag]

Local verification on fix-998-list-resources-handler (855cf72):

go test ./... -count=1
ok  github.com/modelcontextprotocol/go-sdk/mcp
ok  github.com/modelcontextprotocol/go-sdk/oauthex

go test -race ./mcp/... -run TestListResourcesHandler -count=1
ok  github.com/modelcontextprotocol/go-sdk/mcp

I kept resources/directory/read out of scope per the PR note until custom methods (#956) land. Open to splitting static+dynamic pagination into a follow-up if maintainers want a smaller first merge.