Handshake test parses multiple valid RPC messages as one JSON object #640

Problem

JsonRpcServer.EmitsHandshakeBeforeReadingInput parses the entire stdout buffer as one JSON object. The live server can emit a second event after the handshake while the test closes empty input, making valid newline-delimited output fail with Unexpected trailing JSON data. This recurred twice in native-metal-macos CI for #639.

Done when

• Parse the first complete newline-delimited message and assert it is the handshake.
• Retain an assertion that handshake precedes any other output.
• Native macOS CI passes without relying on event timing.