Add MCP Streamable HTTP specification support for the client

[keisku]

Motivation and Context

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

Rebased from #210 (original by @keisku) with merge conflict resolution, bug fixes, and review feedback addressed.

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 via event_stream_parser gem
• Session management (MCP-Session-Id header)
• Handle 404 as session expiration
• Handle 202 Accepted when server delivers response via SSE stream
• DELETE for session termination
• Protocol version header (MCP-Protocol-Version)
• Client conformance test runner (conformance/client.rb)

How Has This Been Tested?

Unit tests — 59 tests pass (40 client + 19 HTTP transport):

ruby -I lib -I test test/mcp/client_test.rb      # 40 runs, 125 assertions, 0 failures
ruby -I lib -I test test/mcp/client/http_test.rb  # 19 runs, 48 assertions, 0 failures

Conformance tests — core client scenarios pass:

npx @modelcontextprotocol/conformance client --suite core --command "ruby conformance/client.rb"

✓ initialize: 1 passed, 0 failed
✓ tools_call: 1 passed, 0 failed

E2E — manual testing with examples/streamable_http_server.rb and examples/streamable_http_client.rb.

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

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)

[keisku]

@atesgoral Thanks for reviewing this! I've addressed it.

[keisku]

@atesgoral Any chance you could take a look at this PR?

[koic]

@keisku Could you review the feedback and squash the commits into a single commit?

[atesgoral]

Commits:

1. Squashed and rebased original PR while also matching the code base's new stylistic preferences
2. Fixed an issue
3. Addressed review comments