Reduce Codex request amplification for large payloads

Reuse immutable request bytes during input inspection and avoid a redundant websocket clone while preserving the required outbound envelope.

Constraint: Preserve request immutability and one final WebSocket request-body allocation
Rejected: In-place JSON mutation | request buffers are shared across translation and execution stages
Confidence: high
Scope-risk: narrow
Directive: Do not retain no-copy gjson results or mutate their backing payload while results are in use
Tested: GOTOOLCHAIN=local go test ./...; targeted 8 MiB allocation benchmarks
Not-tested: Live Codex upstream network traffic