[scheduler] Replace unique_ptr with raw pointers, add leak detection

The scheduler was already managing SchedulerItem lifecycle explicitly
through its object pool (recycle_item_main_loop_ / get_item_from_pool_locked_).
The unique_ptr wrapper added overhead (11 destructor call sites on the hot path)
without providing safety — if a lifecycle path was missed, the unique_ptr would
silently delete the item and cause needless heap allocations instead of pool reuse.

Replace unique_ptr<SchedulerItem, SchedulerItemDeleter> with raw SchedulerItem*
throughout. Every item is now explicitly recycled to the pool or deleted via
delete_item_(). This eliminates all 11 unique_ptr destructor calls from the hot
path and saves ~256 bytes of firmware.

Add debug leak detection under ESPHOME_DEBUG_SCHEDULER: a live-item counter
verified at the end of every call() cycle asserts that all allocated items are
accounted for in items_, to_add_, defer_queue_, or the pool. This turns silent
heap churn from missed lifecycle management into an immediate assert failure
caught by integration tests.

Also moves the retry-cancelled check before item allocation in set_timer_common_
to avoid needless alloc+delete on the cold retry path, and fixes a thread-safety
issue where recycle_item_main_loop_ (main-loop-only) was called from
set_timer_common_ which can run on non-main-loop threads.

Enable debug_scheduler: true in all 18 scheduler integration test fixtures.

tests/integration/fixtures/scheduler_bulk_cleanup.yaml

@@ -1,4 +1,5 @@
 esphome:
+  debug_scheduler: true  # Enable scheduler leak detection
   name: scheduler-bulk-cleanup
 
 external_components:

tests/integration/fixtures/scheduler_defer_cancel.yaml

@@ -1,4 +1,5 @@
 esphome:
+  debug_scheduler: true  # Enable scheduler leak detection
   name: scheduler-defer-cancel
 
 host:

tests/integration/fixtures/scheduler_defer_cancels_regular.yaml

@@ -1,4 +1,5 @@
 esphome:
+  debug_scheduler: true  # Enable scheduler leak detection
   name: scheduler-defer-cancel-regular
 
 host:

tests/integration/fixtures/scheduler_defer_fifo_simple.yaml

@@ -1,4 +1,5 @@
 esphome:
+  debug_scheduler: true  # Enable scheduler leak detection
   name: scheduler-defer-fifo-simple
 
 host:

tests/integration/fixtures/scheduler_defer_stress.yaml

@@ -1,4 +1,5 @@
 esphome:
+  debug_scheduler: true  # Enable scheduler leak detection
   name: scheduler-defer-stress-test
 
 external_components:

tests/integration/fixtures/scheduler_heap_stress.yaml

@@ -1,4 +1,5 @@
 esphome:
+  debug_scheduler: true  # Enable scheduler leak detection
   name: scheduler-heap-stress-test
 
 external_components:

tests/integration/fixtures/scheduler_internal_id_no_collision.yaml

@@ -1,4 +1,5 @@
 esphome:
+  debug_scheduler: true  # Enable scheduler leak detection
   name: scheduler-internal-id-test
   on_boot:
     priority: -100

tests/integration/fixtures/scheduler_null_name.yaml

@@ -1,4 +1,5 @@
 esphome:
+  debug_scheduler: true  # Enable scheduler leak detection
   name: scheduler-null-name
 
 host:

tests/integration/fixtures/scheduler_numeric_id_test.yaml

@@ -1,4 +1,5 @@
 esphome:
+  debug_scheduler: true  # Enable scheduler leak detection
   name: scheduler-numeric-id-test
   on_boot:
     priority: -100

tests/integration/fixtures/scheduler_rapid_cancellation.yaml

@@ -1,4 +1,5 @@
 esphome:
+  debug_scheduler: true  # Enable scheduler leak detection
   name: sched-rapid-cancel-test
 
 external_components:

tests/integration/fixtures/scheduler_recursive_timeout.yaml

@@ -1,4 +1,5 @@
 esphome:
+  debug_scheduler: true  # Enable scheduler leak detection
   name: sched-recursive-timeout
 
 external_components:

tests/integration/fixtures/scheduler_removed_item_race.yaml

@@ -1,4 +1,5 @@
 esphome:
+  debug_scheduler: true  # Enable scheduler leak detection
   name: scheduler-removed-item-race
 
 host:

tests/integration/fixtures/scheduler_retry_test.yaml

@@ -1,4 +1,5 @@
 esphome:
+  debug_scheduler: true  # Enable scheduler leak detection
   name: scheduler-retry-test
   on_boot:
     priority: -100

tests/integration/fixtures/scheduler_simultaneous_callbacks.yaml

@@ -1,4 +1,5 @@
 esphome:
+  debug_scheduler: true  # Enable scheduler leak detection
   name: sched-simul-callbacks-test
 
 external_components:

tests/integration/fixtures/scheduler_string_lifetime.yaml

@@ -1,4 +1,5 @@
 esphome:
+  debug_scheduler: true  # Enable scheduler leak detection
   name: scheduler-string-lifetime-test
 
 external_components:

tests/integration/fixtures/scheduler_string_name_stress.yaml

@@ -1,4 +1,5 @@
 esphome:
+  debug_scheduler: true  # Enable scheduler leak detection
   name: sched-string-name-stress
 
 external_components: