fix(sessions): Top tools counts each tool call once

The session analysis page's Top tools list counted every event carrying a
tool_name, so each call was counted twice (its PreToolUse and its
PostToolUse) while the Tool calls total beside it counted once — a session
with 840 calls showed Bash at 1,533. The list now counts the same way as
the total: PreToolUse, or PostToolUse in a session with none (imported).

server/__tests__/ui-audit-routes.test.js

@@ -93,6 +93,12 @@ describe("GET /api/sessions/:id/stats tool totals", () => {
     assert.equal(body.tool_call_count, 2);
     assert.equal(body.distinct_tool_count, 1);
   });
+  it("counts each call once in tools_used, not its Pre and Post events", async () => {
+    const one = (await get("/api/sessions/ua-1/stats")).body.tools_used;
+    assert.ok(one.every((t) => t.count === 1));
+    const two = (await get("/api/sessions/ua-2/stats")).body.tools_used;
+    assert.deepEqual(two, [{ tool_name: "Read", count: 2 }]);
+  });
 });
 
 describe("GET /api/events labels", () => {