Support client-side elicitation with SEP-1034 schema defaults

## Motivation and Context

Resolves the `elicitation-sep1034-client-defaults` client conformance scenario
(SEP-1034, https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1034).

The Ruby HTTP client could not receive server-to-client requests at all:
it never opened the spec's standalone GET SSE listening stream, and SSE events that
were not responses were discarded. A server sending `elicitation/create` during a tools/call
(such as the conformance harness, which is built on the TypeScript SDK server)
either dropped the request silently or deadlocked the tool call waiting for an answer that
could never arrive.

This change adds three pieces:

### Server-to-client request dispatch

`MCP::Client::HTTP#on_server_request(method) { |params| ... }` registers a handler;
when a JSON-RPC request (a message with both `method` and `id`) arrives on any SSE stream,
the handler's return value is POSTed back to the server as the JSON-RPC `result`
(the server ACKs with 202 Accepted, per the transport spec).
Methods without a handler are answered with a `-32601` method-not-found error.
A handler may raise `MCP::Client::ServerRequestError` to answer with a specific JSON-RPC error code
(for example `-1` for a rejected sampling request, per the sampling spec),
mirroring the TypeScript SDK's `McpError` and the Python SDK's `ErrorData` return;
any other error is answered with a `-32603` internal error. In every case the server is not left waiting,
matching how the TypeScript and Python SDKs convert handler failures into JSON-RPC error responses.

### A standalone GET SSE listening stream

Registering a handler (once connected) opens the spec's listening stream on a background thread,
because servers deliver requests that are not tied to a client request on that stream -
the TypeScript SDK's `StreamableHTTPServerTransport` silently drops them when no GET stream is open.
Mirrors the TypeScript and Python SDK clients, which start listening right after the `initialize` handshake
without awaiting stream establishment. Reconnection semantics also follow both SDKs:
a gracefully closed stream is retried indefinitely after the server's `retry:` interval,
while consecutive connection failures (HTTP errors and idle read timeouts) stop the listener once
they reach the reconnection attempt cap, with a successful stream resetting the count;
a 405 stops immediately without retrying, since it is the spec's answer from a server that
offers no listening stream. The listening GET uses a 5-minute read timeout matching
the Python SDK's `sse_read_timeout` default. `close` terminates the thread.

### SEP-1034 defaults

`MCP::Client::Elicitation.apply_defaults` fills fields omitted from the elicitation content with
the `default` values declared in `requestedSchema` properties, never overwriting provided values
and skipping properties without a default. This follows the Python SDK, which applies defaults in
the elicitation callback; the TypeScript SDK's automatic `applyElicitationDefaults`
(gated on the `elicitation.form.applyDefaults` capability) is a possible follow-up.
`MCP::Client#on_elicitation` registers a handler for `elicitation/create` as a convenience.

The conformance client gains an `elicitation-sep1034-client-defaults` branch that connects with
`capabilities: { elicitation: {} }`, accepts the elicitation with defaults applied, and calls
the harness's `test_client_elicitation_defaults` tool; the scenario is removed from the expected failures baseline.

## How Has This Been Tested?

New unit tests covering: handler dispatch from a request's POST SSE stream with
the response body and 202 ACK verified, the `-32601` answer for unregistered methods,
the `-32603` answer for a raising handler, the custom-code answer when a handler raises
`ServerRequestError`, listener startup from both registration-after-connect and
connect-with-registered-handlers, listener shutdown after consecutive connection failures with
the count resetting after a successful stream and an immediate stop on 405,
and `apply_defaults` for all five SEP-1034 property types (string, integer, number, enum, boolean),
non-overwriting of provided values, omission of properties without defaults, and symbol-keyed schemas

## Breaking Changes

None. The listening stream and its background thread are created only when a server-request handler is registered;
clients that never call `on_server_request` / `on_elicitation` keep the existing single-request behavior.

conformance/client.rb

@@ -125,7 +125,11 @@ end
 oauth = scenario.start_with?("auth/") ? build_provider_for(scenario, conformance_context) : nil
 transport = MCP::Client::HTTP.new(url: server_url, oauth: oauth)
 client = MCP::Client.new(transport: transport)
-client.connect(client_info: { name: "ruby-sdk-conformance-client", version: MCP::VERSION })
+capabilities = scenario == "elicitation-sep1034-client-defaults" ? { elicitation: {} } : {}
+client.connect(
+  client_info: { name: "ruby-sdk-conformance-client", version: MCP::VERSION },
+  capabilities: capabilities,
+)
 
 case scenario
 when "initialize"
@@ -144,6 +148,19 @@ when "sse-retry"
   test_reconnection = tools.find { |t| t.name == "test_reconnection" }
   abort("Tool test_reconnection not found") unless test_reconnection
   client.call_tool(tool: test_reconnection, arguments: {})
+when "elicitation-sep1034-client-defaults"
+  # SEP-1034: the server sends `elicitation/create` on the tools/call SSE stream with every property declaring
+  # a `default` and none required. The handler accepts and fills the omitted fields from those defaults.
+  client.on_elicitation do |params|
+    { action: "accept", content: MCP::Client::Elicitation.apply_defaults(params["requestedSchema"] || {}) }
+  end
+
+  tools = client.tools
+  defaults_tool = tools.find { |t| t.name == "test_client_elicitation_defaults" }
+
+  abort("Tool test_client_elicitation_defaults not found") unless defaults_tool
+
+  client.call_tool(tool: defaults_tool, arguments: {})
 when %r|\Aauth/|
   # Auth-only scenarios: the protocol-level checks (PRM/AS metadata, DCR, PKCE, token usage)
   # are observed by the conformance server during `connect` and the subsequent request below.

conformance/expected_failures.yml

@@ -1,7 +1,5 @@
 server: []
 client:
-  # TODO: Elicitation not implemented in Ruby client.
-  - elicitation-sep1034-client-defaults
   # TODO: Remaining OAuth/auth scenarios not yet implemented in Ruby client.
   - auth/client-credentials-jwt
   - auth/cross-app-access-complete-flow