Support Capability Extensions per SEP-2133

[koic]

Motivation and Context

SEP-2133 (modelcontextprotocol/modelcontextprotocol#2133, merged for the 2026-07-28 spec release) establishes the extensions framework for MCP: both ClientCapabilities and ServerCapabilities gain an optional extensions member keyed by reverse-DNS extension identifiers
(e.g. "io.modelcontextprotocol/tasks", "com.example/feature"), with arbitrary extension-defined configuration objects as values and an empty object meaning "supported with no settings". Official extensions such as MCP Apps (SEP-1865) and the Tasks extension (SEP-2663) are layered on this field.

This mirrors the shape the TypeScript SDK merged (typescript-sdk#1630): plain declaration and passthrough, no registrar API and no key-format validation (the naming convention is documented instead).

• MCP::Server::Capabilities gains support_extensions (repeated calls merge), accepts an :extensions key in its constructor hash, and includes extensions in to_h. The class was previously dead code: it was never required and Server.new only took plain hashes. It is now required by lib/mcp/server.rb, and Server.new(capabilities:) accepts either a plain Hash (as before) or a Capabilities instance.
• Server-declared extensions appear in the initialize result; extensions the client declares during initialize were already stored verbatim and are readable via Server#client_capabilities[:extensions] and ServerSession#client_capabilities[:extensions] (now covered by tests).
• MCP::Client#connect(capabilities:) already passes capabilities through verbatim; its documentation now covers the extensions member.

Resolves #378.

How Has This Been Tested?

• New test/mcp/server/capabilities_test.rb covers the previously untested Capabilities class: empty default, constructor round-trip, extensions via constructor and support_extensions, merge semantics, nil tolerance, and omission from to_h when never declared.
• New tests in test/mcp/server_test.rb assert that the initialize result carries declared extensions (from both a plain hash and a Capabilities instance) and that client-declared extensions are readable via Server#client_capabilities and ServerSession#client_capabilities.

Breaking Changes

None. extensions is additive and appears on the wire only when declared; servers constructed with plain capability hashes behave 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