Validate MCP-Protocol-Version header in StreamableHTTPTransport

## Motivation and Context

MCP 2025-11-25 (basic/transports#protocol-version-header) requires servers
to respond with `400 Bad Request` when receiving a request with an invalid
or unsupported `MCP-Protocol-Version` header:

> If the server receives a request with an invalid or unsupported
> `MCP-Protocol-Version`, it MUST respond with `400 Bad Request`.

The Ruby SDK was not reading the header at all, accepting any value
(including malformed strings like `not-a-version` and unsupported versions
like `1900-01-01`) and dispatching requests normally with HTTP 200.

## Behavior

`StreamableHTTPTransport` now validates the `MCP-Protocol-Version` header
on POST (non-initialize), GET, and DELETE requests. Missing headers are
accepted as before; the spec SHOULD requirement to default to `2025-03-26`
is intentionally left for a follow-up. The `initialize` POST is exempt
because the client does not know the negotiated version until the response
arrives, matching the Python (`src/mcp/server/streamable_http.py`) and
TypeScript (`packages/server/src/server/streamableHttp.ts`) SDKs.

The error response uses a JSON-RPC envelope with `code: -32600`
(`INVALID_REQUEST`) to match the Python SDK shape:

```json
{
  "jsonrpc": "2.0",
  "id": null,
  "error": {
    "code": -32600,
    "message": "Bad Request: Unsupported protocol version: <value>. Supported versions: ..."
  }
}
```

Validation order is session then protocol version, matching the Python and
TypeScript SDKs.

`handle_post` was reorganized so that non-Hash request bodies (e.g., JSON
arrays, which MCP 2025-11-25 no longer supports as batches) also pass
through the header check. The pre-existing broken response for Array
bodies sent with a valid header is unchanged and is tracked as a separate
follow-up.

## How Has This Been Tested?

Added regression tests covering:

- POST `initialize` ignores the header (bypass)
- POST non-initialize with unsupported / malformed / valid / missing values
- POST array body with unsupported value returns 400
- GET with unsupported / missing values
- DELETE with unsupported value in both stateful and stateless modes
- DELETE validates session before protocol version

`bundle exec rake test` and `bundle exec rake rubocop` both pass.

## Breaking Changes

Strictly additive for compliant clients. Third-party clients sending stale
or malformed `MCP-Protocol-Version` values will now receive `400` instead
of `200`, which is the intended spec behavior.

Fixes #346.

lib/mcp/server/transports/streamable_http_transport.rb

@@ -340,23 +340,28 @@ module MCP
           session_id = extract_session_id(request)
 
           body = parse_request_body(body_string)
-          return body unless body.is_a?(Hash) # Error response
+          return body if parse_error_tuple?(body)
 
-          if body[:method] == "initialize"
-            handle_initialization(body_string, body)
-          else
+          unless initialize_request?(body)
             return missing_session_id_response if !@stateless && !session_id
 
-            if notification?(body)
-              dispatch_notification(body_string, session_id)
-              handle_accepted
-            elsif response?(body)
-              return session_not_found_response if !@stateless && !session_exists?(session_id)
+            protocol_version_error = validate_protocol_version_header(request)
+            return protocol_version_error if protocol_version_error
+          end
 
-              handle_response(body, session_id: session_id)
-            else
-              handle_regular_request(body_string, session_id, related_request_id: body[:id])
-            end
+          return body unless body.is_a?(Hash) # Non-Hash JSON-RPC bodies are not supported in 2025-11-25.
+
+          if initialize_request?(body)
+            handle_initialization(body_string, body)
+          elsif notification?(body)
+            dispatch_notification(body_string, session_id)
+            handle_accepted
+          elsif response?(body)
+            return session_not_found_response if !@stateless && !session_exists?(session_id)
+
+            handle_response(body, session_id: session_id)
+          else
+            handle_regular_request(body_string, session_id, related_request_id: body[:id])
           end
         rescue StandardError => e
           MCP.configuration.exception_reporter.call(e, { request: body_string })
@@ -377,6 +382,10 @@ module MCP
 
           error_response = validate_and_touch_session(session_id)
           return error_response if error_response
+
+          protocol_version_error = validate_protocol_version_header(request)
+          return protocol_version_error if protocol_version_error
+
           return session_already_connected_response if get_session_stream(session_id)
 
           setup_sse_stream(session_id)
@@ -386,6 +395,9 @@ module MCP
           success_response = [200, { "Content-Type" => "application/json" }, [{ success: true }.to_json]]
 
           if @stateless
+            protocol_version_error = validate_protocol_version_header(request)
+            return protocol_version_error if protocol_version_error
+
             # Stateless mode doesn't support sessions, so we can just return a success response
             return success_response
           end
@@ -393,6 +405,9 @@ module MCP
           return missing_session_id_response unless (session_id = request.env["HTTP_MCP_SESSION_ID"])
           return session_not_found_response unless session_exists?(session_id)
 
+          protocol_version_error = validate_protocol_version_header(request)
+          return protocol_version_error if protocol_version_error
+
           cleanup_session(session_id)
 
           success_response
@@ -495,6 +510,31 @@ module MCP
           [400, { "Content-Type" => "application/json" }, [{ error: "Invalid JSON" }.to_json]]
         end
 
+        def parse_error_tuple?(body)
+          body.is_a?(Array) && body.size == 3 && body[0] == 400
+        end
+
+        def initialize_request?(body)
+          body.is_a?(Hash) && body[:method] == Methods::INITIALIZE
+        end
+
+        def validate_protocol_version_header(request)
+          header_value = request.env["HTTP_MCP_PROTOCOL_VERSION"]
+          return if header_value.nil?
+          return if MCP::Configuration::SUPPORTED_STABLE_PROTOCOL_VERSIONS.include?(header_value)
+
+          supported = MCP::Configuration::SUPPORTED_STABLE_PROTOCOL_VERSIONS.join(", ")
+          body = {
+            jsonrpc: "2.0",
+            id: nil,
+            error: {
+              code: JsonRpcHandler::ErrorCode::INVALID_REQUEST,
+              message: "Bad Request: Unsupported protocol version: #{header_value}. Supported versions: #{supported}",
+            },
+          }
+          [400, { "Content-Type" => "application/json" }, [body.to_json]]
+        end
+
         def notification?(body)
           !body[:id] && !!body[:method]
         end