Support Capability Extensions per SEP-2133

## 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.

lib/mcp/server.rb

@@ -8,6 +8,7 @@ require_relative "methods"
 require_relative "logging_message_notification"
 require_relative "progress"
 require_relative "server_context"
+require_relative "server/capabilities"
 require_relative "server/pagination"
 require_relative "server/transports"
 
@@ -142,7 +143,12 @@ module MCP
 
       validate!
 
-      @capabilities = capabilities || default_capabilities
+      # Accept either a plain Hash or an `MCP::Server::Capabilities` builder.
+      @capabilities = if capabilities.is_a?(Capabilities)
+        capabilities.to_h
+      else
+        capabilities || default_capabilities
+      end
       @client_capabilities = nil
       @logging_message_notification = nil