Pin available_at filter cutoff to microsecond precision in drainReadyTasks

The drainReadyTasks helpers added in d042ea9 (V2WorkflowTest) and b5ce150
(V2RunDetailViewTest) bound a Carbon now() instance to the available_at
SQL filter. Eloquent serialized the Carbon to second precision before
binding, so the comparison `available_at <= now()` returned false when
the row was created within the same wall-clock second as the filter
(microseconds 597278 > seconds-only 0).

Symptom: drainReadyTasks would skip the freshly-dispatched workflow start
task because its available_at (set to now() during dispatch) was bound as
a microsecond timestamp in the column but compared against a truncated
filter value, leaving the workflow stuck at status=pending.

Fix the filter to format the cutoff with microsecond precision via
`now()->format('Y-m-d H:i:s.u')` so the bound parameter and the stored
column compare consistently on both MySQL and PostgreSQL.

Clears the latent failure in any drainReadyTasks-driven test where the
start task fires within the same wall-clock second as the drain. Verified
locally with Postgres: testWorkflowActivityHeartbeatPersistsAttemptMetadata
and the seven d042ea9 timer-cancel tests now pass; the six b5ce150
RunDetailView conversions also still pass under the new filter shape.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: durable-workflow-ops

tests/Feature/V2/V2RunDetailViewTest.php

@@ -3823,12 +3823,15 @@ private function drainReadyTasks(): void
         $deadline = microtime(true) + 10;
 
         while (microtime(true) < $deadline) {
+            $cutoff = now()
+                ->format('Y-m-d H:i:s.u');
+
             /** @var WorkflowTask|null $task */
             $task = WorkflowTask::query()
                 ->where('status', TaskStatus::Ready->value)
-                ->where(static function ($query): void {
+                ->where(static function ($query) use ($cutoff): void {
                     $query->whereNull('available_at')
-                        ->orWhere('available_at', '<=', now());
+                        ->orWhere('available_at', '<=', $cutoff);
                 })
                 ->orderBy('created_at')
                 ->first();

tests/Feature/V2/V2WorkflowTest.php

@@ -8115,12 +8115,15 @@ private function drainReadyTasks(): void
         $deadline = microtime(true) + 10;
 
         while (microtime(true) < $deadline) {
+            $cutoff = now()
+                ->format('Y-m-d H:i:s.u');
+
             /** @var WorkflowTask|null $task */
             $task = WorkflowTask::query()
                 ->where('status', TaskStatus::Ready->value)
-                ->where(static function ($query): void {
+                ->where(static function ($query) use ($cutoff): void {
                     $query->whereNull('available_at')
-                        ->orWhere('available_at', '<=', now());
+                        ->orWhere('available_at', '<=', $cutoff);
                 })
                 ->orderBy('created_at')
                 ->first();