[ejgallego]

This PR returns a correlated structured error when an oversized broker result or validation reply is rejected before any frame bytes are written. Previously, a completed result such as a large hover could surface as brokerUnavailable even though the Lean backend remained healthy.

• Prepare size-checked frame bytes before changing output state, including greetings and recovery replies; preserve connection abandonment after a send begins.
• Preserve request correlation in validation-error recovery and retain the machine-readable operation_may_have_completed warning only on result-delivery failures.
• Cover byte boundaries, committed effects, stopped-reader send deadlines, and raw-protocol recovery followed by strict EOF.