## 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 g`max_message_bytes:` to accept them.
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