[core] Replace scheduler pool vector with unbounded intrusive freelist (#16172)

tests/integration/fixtures/scheduler_pool.yaml

@@ -221,14 +221,10 @@ script:
   - id: test_full_pool_reuse
     then:
       - lambda: |-
-          ESP_LOGI("test", "Phase 6: Testing pool size limits after Phase 5 items complete");
+          ESP_LOGI("test", "Phase 6: Testing pool reuse after Phase 5 items complete");
 
-          // At this point, all Phase 5 timeouts should have completed and been recycled.
-          // The pool should be at its maximum size (5).
-          // Creating 10 new items tests that:
-          // - First 5 items reuse from the pool
-          // - Remaining 5 items allocate new (pool empty)
-          // - Pool doesn't grow beyond MAX_POOL_SIZE of 5
+          // Phase 5 timeouts have completed and been recycled. The freelist is unbounded;
+          // creating 10 new items reuses from it and only allocates fresh when empty.
 
           auto *component = id(test_sensor);
           int full_reuse_count = 10;

tests/integration/test_scheduler_pool.py

@@ -180,16 +180,22 @@ async def test_scheduler_pool(
     # Verify pool behavior
     assert pool_recycle_count > 0, "Should have recycled items to pool"
 
-    # Check pool metrics
-    if pool_recycle_count > 0:
-        max_pool_size = 0
-        for line in log_lines:
-            if match := recycle_pattern.search(line):
-                size = int(match.group(1))
-                max_pool_size = max(max_pool_size, size)
+    # Pool is unbounded; the cap was the source of the churn it was meant to prevent.
+    assert pool_full_count == 0, (
+        f"Pool should never report full (got {pool_full_count})"
+    )
 
-        # Pool can grow up to its maximum of 5
-        assert max_pool_size <= 5, f"Pool grew beyond maximum ({max_pool_size})"
+    # Verify the pool actually grew past the old MAX_POOL_SIZE=5 cap.
+    # Phase 5 + Phase 6 schedule 8 + 10 same-component timeouts respectively, so the
+    # observed peak should comfortably exceed 5. Without this lower-bound check, a
+    # silent regression that re-introduced a small cap could pass the test above.
+    max_pool_size = 0
+    for line in log_lines:
+        if match := recycle_pattern.search(line):
+            max_pool_size = max(max_pool_size, int(match.group(1)))
+    assert max_pool_size > 5, (
+        f"Pool should grow past the old cap of 5; observed peak {max_pool_size}"
+    )
 
     # Log summary for debugging
     print("\nScheduler Pool Test Summary (Python Orchestrated):")