Add a resources_list_handler for context-dependent resource lists

[koic]

Motivation and Context

resources/read can be taken over with resources_read_handler, but resources/list had no equivalent: it was bound to the internal list_resources, which paginates the constructor-provided resources array, and define_custom_method refuses to rebind a method the server already handles. An application whose visible resources depend on the request - authenticated versus anonymous sessions, or an OAuth-scoped connector that must see a scope-filtered set - had to build a fresh Server per request or override a private method.

Both reference SDKs let the list vary per request. The TypeScript SDK exposes resources/list through setRequestHandler and gives resource templates a per-context listCallback(ctx); the Python SDK takes an on_list_resources handler. This adds the Ruby equivalent.

resources_list_handler(&block) registers a block that returns the resource collection to serve. It feeds list_resources rather than replacing it, so the framework still paginates the returned array and stamps the SEP-2549 cache hints; the block returns only the array, matching how resources_read_handler returns only the contents and the framework wraps them. A block that declares a server_context: keyword receives an MCP::ServerContext, reusing the same opt-in rule the read handler uses, so the list can be filtered by the authenticated principal or granted scope. The block is invoked once per page and must return a stable ordering across the pages of one query, since the cursor is a positional offset. When no handler is set, the constructor-provided resources are served unchanged.

The resources capability is advertised whether or not the constructor array is populated, so a server that provides only a resources_list_handler still answers resources/list.

Fixes #507.

How Has This Been Tested?

New tests in test/mcp/server_test.rb cover a handler replacing the served collection, a handler receiving server_context: and returning different sets for authenticated and anonymous requests, pagination and cache hints still applying to the handler-provided array, and a server with no constructor-provided resources served entirely by the handler. The existing resources/list test, which exercises the no-handler default path, is unchanged.

Breaking Changes

None. When no resources_list_handler is set, resources/list serves the constructor-provided resources exactly as before.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed