Add MCP Streamable HTTP specification support for the client

[keisku]

Motivation and Context

Implements the MCP Streamable HTTP specification for the Ruby SDK client.

Already supported:

• POST with JSON body
• Accept: application/json, text/event-stream header
• Parse application/json response

This PR adds:

• Parse text/event-stream (SSE) response
• Session management (MCP-Session-Id header)
• Handle 404 as session expiration
• DELETE for session termination
• Protocol version header (MCP-Protocol-Version)

How Has This Been Tested?

Terminal 1 - Server

bundle exec ruby ./examples/streamable_http_server.rb

# After client requests...

[MCP] Request: initialize (id: 7bb764c8-cd31-4ca8-8073-a7fc977a53dd)
[SSE] INFO 23:03:05.053 - New client initializing session
[MCP] Response: success (id: 7bb764c8-cd31-4ca8-8073-a7fc977a53dd)
[SSE] INFO 23:03:05.053 - Session created: 6e58438d-ca95-47d3-b2cd-4df7c6915dcf
::1 - - [09/Jan/2026:23:03:05 +0900] "POST / HTTP/1.1" 200 - 0.0003
[MCP] Request: tools/list (id: d2cc3984-cbf4-4e31-9513-074b36c7dab5)
[MCP] Response: success (id: d2cc3984-cbf4-4e31-9513-074b36c7dab5)
::1 - - [09/Jan/2026:23:03:05 +0900] "POST / HTTP/1.1" 200 - 0.0001
[SSE] INFO 23:03:05.055 - SSE connection request for session: 6e58438d-ca95-47d3-b2cd-4df7c6915dcf
[SSE] INFO 23:03:05.055 - SSE stream established
::1 - - [09/Jan/2026:23:03:05 +0900] "GET / HTTP/1.1" 200 - 0.0001
[MCP] Request: tools/call (id: 822967ca-da23-484f-86b3-caaef42e4209)
[SSE] INFO 23:03:17.068 - Returning notification message: HELLO
[SSE] INFO 23:03:17.068 - Response sent via SSE stream
::1 - - [09/Jan/2026:23:03:17 +0900] "POST / HTTP/1.1" 200 - 3.0057
[MCP] Request: tools/call (id: fa2ad1e5-1fef-4d02-b20e-3f1ae9425bd0)
[SSE] INFO 23:03:24.339 - Response sent via SSE stream
::1 - - [09/Jan/2026:23:03:24 +0900] "POST / HTTP/1.1" 200 - 0.0005
[MCP] Request: tools/list (id: 6d532649-b594-4f7f-bfce-ebb091015e6b)
[SSE] INFO 23:03:26.380 - Response sent via SSE stream
::1 - - [09/Jan/2026:23:03:26 +0900] "POST / HTTP/1.1" 200 - 0.0002
::1 - - [09/Jan/2026:23:03:27 +0900] "DELETE / HTTP/1.1" 200 - 0.0001

Terminal 2 - Client

bundle exec ruby examples/streamable_http_client.rb
MCP Streamable HTTP Client (SDK + SSE)
Make sure the server is running (ruby examples/streamable_http_server.rb)
============================================================
=== Initializing session ===
Session ID: 6e58438d-ca95-47d3-b2cd-4df7c6915dcf
Protocol Version: 2025-11-25
Server Info: {"icons" => [], "name" => "sse_test_server", "version" => "0.1.0"}
=== Listing tools ===
  - notification_tool: Returns a notification message that will be sent via SSE if stream is active
  - echo: Simple echo tool
[CLIENT] INFO 23:03:05.054 - Connecting to SSE stream...
[CLIENT] INFO 23:03:05.055 - SSE stream connected successfully

=== Available Actions ===
1. Send notification (triggers SSE event)
2. Echo message
3. List tools
0. Exit

Choose an action:
1
Enter notification message: HELLO
Enter delay in seconds (0 for immediate): 3
=== Calling tool: notification_tool ===
[CLIENT] INFO 23:03:17.068 - SSE event: {"jsonrpc":"2.0","id":"822967ca-da23-484f-86b3-caaef42e4209","result":{"content":[{"type":"text","text":"Notification: HELLO (timestamp: 2026-01-09T23:03:17+09:00)"}],"isError":false}}
Response: {
  "accepted": true
}

=== Available Actions ===
1. Send notification (triggers SSE event)
2. Echo message
3. List tools
0. Exit

Choose an action:
2
Enter message to echo: HELOOOOOOO
=== Calling tool: echo ===
[CLIENT] INFO 23:03:24.339 - SSE event: {"jsonrpc":"2.0","id":"fa2ad1e5-1fef-4d02-b20e-3f1ae9425bd0","result":{"content":[{"type":"text","text":"Echo: HELOOOOOOO"}],"isError":false}}
Response: {
  "accepted": true
}

=== Available Actions ===
1. Send notification (triggers SSE event)
2. Echo message
3. List tools
0. Exit

Choose an action:
3
=== Listing tools ===
(Note: Response will appear in SSE stream when active)
[CLIENT] INFO 23:03:26.380 - SSE event: {"jsonrpc":"2.0","id":"6d532649-b594-4f7f-bfce-ebb091015e6b","result":{"tools":[{"name":"notification_tool","description":"Returns a notification message that will be sent via SSE if stream is active","inputSchema":{"properties":{"message":{"type":"string","description":"Message to send via SSE"},"delay":{"type":"number","description":"Delay in seconds before returning (optional)"}},"required":["message"],"type":"object"}},{"name":"echo","description":"Simple echo tool","icons":[],"inputSchema":{"properties":{"message":{"type":"string"}},"required":["message"],"type":"object"}}]}}

=== Available Actions ===
1. Send notification (triggers SSE event)
2. Echo message
3. List tools
0. Exit

Choose an action:
0
[CLIENT] INFO 23:03:27.995 - Exiting...
=== Closing session ===
Session closed

Breaking Changes

None. The public send_request(request:) method signature is preserved for backward compatibility.

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

Additional context

API Design

• MCP::Client::HTTP#send_request(request:) - Returns body only (backward compatible)
• MCP::Client::HTTP#post(body:, headers:) - Returns Response struct with body + headers (for session management)
• MCP::Client::HTTP#delete(headers:) - For session termination
• MCP::Client#connect(client_info:, protocol_version:, capabilities:) - Initialization handshake
• MCP::Client#close - Session termination

Pending TODOs

• Client GET for SSE stream
• Resumability with Last-Event-ID header
• Origin header validation (server-side)

[atesgoral]

Let's use https://rubygems.org/gems/event_stream_parser for parsing. SSE appears easy to parse but it has weird edge cases.