insight-chat: code-review polish on the days_radius fix

- Bind effective_radius once in fetch_messages_for_contact so the log
  output and window math share a single source of truth for the clamp.
- Clamp tool-supplied days_radius to [1, 30] at the tool boundary so a
  runaway LLM value can't produce a thousand-day window.
- Split the negative-input test into a real negative-input case
  alongside the zero-input case.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ai/insight_generator.rs

@@ -1812,7 +1812,8 @@ Return ONLY the summary, nothing else."#,
         let days_radius = args
             .get("days_radius")
             .and_then(|v| v.as_i64())
-            .unwrap_or(4);
+            .unwrap_or(4)
+            .clamp(1, 30);
         let limit = args
             .get("limit")
             .and_then(|v| v.as_i64())