Bound client-side message buffering in the HTTP transport

[koic]

Motivation and Context

MCP::Client::HTTP buffers server bytes with no upper bound in two places:

• SSE responses are parsed incrementally, but the parser accumulates data: lines until a blank line dispatches the event, so a server that never terminates an event grows the buffer indefinitely. The abort-on-response signal does not help here, since it only fires after a complete response event has been dispatched.
• Non-SSE (JSON) bodies accumulate in SSEStream#buffer chunk by chunk with no limit.

Both the POST response stream and the standalone GET listening stream are affected. The stdio client already bounds server frames (max_line_bytes:, 4 MiB default) and the server transports bound request bodies (max_request_bytes:, 4 MiB default); this brings the HTTP client in line.

MCP::Client::HTTP.new gains max_message_bytes: (default 4 MiB), which caps the bytes buffered for a single message - an SSE event or a JSON response body. External byte counting cannot tell consumed-and-discarded bytes (comments, field names, delimiters) from consumed-and-retained ones, so the check measures the parser's own String buffers after each chunk. An over-limit message raises RequestHandlerError on the request path and stops the GET listening stream instead of reconnecting to the same over-limit event.

How Has This Been Tested?

New tests in test/mcp/client/http_test.rb cover the SSE and JSON rejection paths end-to-end, the per-event reset (events that individually fit must not count against later ones), the env-less on_data fallback, and the constructor validation. bundle exec rake (tests, RuboCop, and conformance baseline) passes.

Breaking Changes

Responses larger than 4 MiB are now rejected by default; pass a larger gmax_message_bytes: to accept them.

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