fix: return 500 for tool execution failures instead of 400

Tool runtime errors are server-side faults, not client input errors.
Returning 400 causes clients to mishandle retries/backoff.

Addresses Greptile review feedback on #13185.

src/gateway/tools-invoke-http.ts

@@ -349,7 +349,7 @@ export async function handleToolsInvokeHttpRequest(
     sendJson(res, 200, { ok: true, result });
   } catch (err) {
     logWarn(`tools-invoke: tool execution failed: ${String(err)}`);
-    sendJson(res, 400, {
+    sendJson(res, 500, {
       ok: false,
       error: { type: "tool_error", message: "tool execution failed" },
     });