fix(hooks): raw hook JSON rendered to terminal when hook output has no message field

Environment

• kimi-code 0.40.1, Windows 11 (also reproducible cross-platform)

Problem

When a UserPromptSubmit hook prints a JSON object that contains only non-message fields (or is empty), the TUI falls back to rendering the raw stdout JSON as a chat message. The user sees the raw JSON blob in the transcript.

Repro

1. Register a UserPromptSubmit hook that prints {} (or JSON without a message field).
2. Send any prompt — the raw JSON appears on screen.

Analysis

Both engine generations (v1 and v2) share the same fallback: when structured-output parsing yields no message, stdout is treated as plain text and injected/rendered verbatim. Hooks that legitimately produce structured-only output cannot avoid leaking JSON to the screen.

Fix

When hook output parses as structured JSON, suppress the plain-stdout fallback. Fixed in both pipelines in our local build; happy to open a PR.

Start at UserPromptSubmit handling in both the v1 and v2 hook pipelines, then trace the structured-output parsing and plain-stdout fallback. Add regression coverage for empty or message-less JSON and verify that structured JSON is not rendered as a chat message while outputs containing a message still appear correctly.