Reject truncated JSON-RPC message bodies

[OskarEichler]

Summary

Verify that the body read returns exactly the byte count declared by Content-Length and raise EOFError when the stream ends early.

Reproduction

IO#read may return a shorter String at EOF. The current reader passes that partial body to JSON parsing, which produces an indirect parser error or can accept a complete shorter JSON value despite the declared frame length. The focused external model fails on 3.17.0.6 and current main, then passes with this change.

Verification

• Existing suite: 6 runs, 11 assertions, zero failures
• Focused truncated-body model passes
• All Ruby syntax checks pass
• Steep passes
• Generated source remains clean
• Rebuilt gem preserves expected package contents
• RuboCop LSP initialize/shutdown integration passes
• Rails 8.1.3.1 loads the candidate

No tests were changed. This source-only patch was identified and verified during an AI-assisted dependency audit.