…sible to LLM

When a tool call requires user approval in the non-blocking gateway path,
the LLM previously received a result that was indistinguishable from a
failed tool call (exit_code=-1, error=message). The LLM could not tell
whether the tool was pending approval, had returned empty results, or had
failed silently — causing it to burn context on wrong hypotheses.

Fix changes the result format to include:
- status: pending_approval (clear state name)
- approval_pending: True (explicit boolean for LLMs to detect)
- error: cleared to empty string (removes misleading error signal)

This lets the LLM reason about approval latency vs actual errors,
short-circuiting the previous silent failure mode.

Fixes #14806