feat(mcp): return structured (JSON) tool output alongside text

## Problem

Tools return combined stdout/stderr as plain text only. The underlying commands already support `-json` (`DatabaseInfo`, `LTXFileInfo`, `DBStatus`, `RestorePlan`/`RestoreResult`). MCP supports `structuredContent` for machine-readable results plus a short text summary for the model.

## Fix

Return `structuredContent` (typed, reusing the existing `-json` output structs) plus a concise text summary from each tool.

## Acceptance

Tools return machine-readable structured output and a human/model-readable summary.

## Cross-refs

After #1368 (uses the official SDK's typed result API).