Parse SSE responses in HTTP client via event_stream_parser

[atesgoral]

Summary

First slice of #321 (Streamable HTTP client support), factored out for easier review.

The Streamable HTTP spec allows servers to respond to a POST with either application/json or text/event-stream. MCP::Client::HTTP currently rejects the latter. This PR adds SSE parsing via the optional event_stream_parser gem — users add it to their Gemfile only when their server uses SSE, matching the existing pattern for faraday.

send_request now dispatches on the response Content-Type, scans the event stream for the first JSON-RPC response message (skipping comments, notifications, and non-response events), and returns it.

This PR intentionally does not include other Streamable HTTP features — those will land as follow-up stacked PRs:

• 202 Accepted handling for async-delivered responses
• Mcp-Session-Id header tracking / SessionExpiredError on 404
• Client#connect handshake helper + protocol_version tracking
• Client#close / DELETE session termination
• Example + docs rewrite

Splitting it this way keeps each PR self-contained and shippable: after this merges, the client correctly handles both JSON and SSE responses from stateless servers.

Changes

• lib/mcp/client/http.rb: replace validate_response_content_type! with a parse_response_body dispatch on Content-Type; add parse_sse_response and require_event_stream_parser!
• Gemfile: add event_stream_parser to the test group
• docs/building-clients.md: mention event_stream_parser as an optional dependency for SSE responses
• test/mcp/client/http_test.rb: add tests for SSE happy path, SSE error response, SSE stream with no response message, and LoadError when the gem is missing; convert the old "non-JSON response" test to use text/html (since text/event-stream is now valid)

Test plan

• rake test passes (737 runs, 1847 assertions)
• rake rubocop shows no new offenses on changed files
• New tests cover SSE happy path, SSE error responses, empty SSE streams, and missing event_stream_parser gem

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