fix: always enable content capture for CLI debug panel (#4581)

Set OTEL_INSTRUMENTATION_GENAI_CAPTURE_MESSAGE_CONTENT=true before
SDK init so the debug panel always shows full prompts, responses,
and tool arguments. Without this, users without the env var set
see empty content in the debug panel.

When user OTel is disabled, SDK spans go to /dev/null so captured
content never leaves the process.