Reuse fixed SQL statements for observation pruning

tests/unit/host-observation-retention.test.ts

@@ -141,6 +141,31 @@ describe('routine host observation retention', () => {
         expect(api.handle.prepare("SELECT COUNT(*) AS count FROM ec_host_operations WHERE kind='instance.restart'").get()).toEqual({ count: 2000 });
     });
 
+    it('reuses the same three delete statements across different batch sizes', async () => {
+        const api = await setup();
+        const statements = new Map<string, ReturnType<typeof api.handle.prepare>>();
+        for (const count of [1, 2, 7, 50, 500]) {
+            api.handle.transaction(() => {
+                const insert = api.handle.prepare(`INSERT INTO ec_host_operations
+                    (id,host_id,kind,payload,state,created_at,updated_at,observation_success)
+                    VALUES(?,?,'files.stat','{}','completed',?,?,1)`);
+                for (let i = 0; i < count; i++) insert.run(`read-${count}-${i}`, api.host.id, at(2 * day), at(2 * day));
+            })();
+            const prepare = vi.spyOn(api.handle, 'prepare');
+            expect(api.pruneHostObservations()).toBe(count);
+            const deletes = prepare.mock.calls.map(([sql]) => sql).filter(sql => sql.startsWith('DELETE FROM ec_host_'));
+            prepare.mockRestore();
+            expect(deletes).toHaveLength(3);
+            for (const sql of deletes) {
+                const statement = api.handle.prepare(sql);
+                if (statements.has(sql)) expect(statement).toBe(statements.get(sql));
+                statements.set(sql, statement);
+            }
+        }
+        expect(statements.size).toBe(3);
+        expect(api.handle.pragma('foreign_key_check')).toEqual([]);
+    });
+
     it('backfills legacy observation classification once while preserving actions and negative or invalid results', async () => {
         const api = await setup();
         const read = api.add('files.manifest');