feat(taskqueue): migrate push queue client to Cloud Tasks v2 SDK

[riddhi-shivhare]

Description

Prepares the App Engine TaskQueue PHP SDK for General Availability (GA) by routing Push Queue operations to the official Google Cloud Tasks v2 client SDK (google/cloud-tasks ^2.0).

Changes Summary

1. Dependency Addition (composer.json):

   • Added "google/cloud-tasks": "^2.0" to require.

2. Dual Environment Variable Routing (PushQueue.php):

   • Added support for routing push tasks via either:
     • GAE_PUSHQUEUE_BACKEND: CLOUD_TASK
     • APPENGINE_USE_CLOUDTASK_PUSH_QUEUE: true / 1

3. Task Creation (PushQueue.php):

   • Single Task Creation: Uses \Google\Cloud\Tasks\V2\Client\CloudTasksClient with \Google\Cloud\Tasks\V2\CreateTaskRequest and AppEngineHttpRequest.
   • Batch Task Creation (addTasks):
     • Automatically inspects the client for native batchCreateTasks support (BatchCreateTasksRequest) for future GA client library updates.
     • Gracefully falls back to sequential createTask calls when batchCreateTasks is not yet generated in the installed google/cloud-tasks package, ensuring reliability across all versions.

4. Request Normalization & Error Mapping:

   • Properly normalizes HTTP method enum integers, AppEngineHttpRequest relative URIs, headers (Content-Type, X-AppEngine-QueueName, X-AppEngine-TaskName), and delay schedules (Google\Protobuf\Timestamp).
   • Maps Cloud Tasks ALREADY_EXISTS (code 6 / 409) and tombstoned task errors to TaskAlreadyExistsException, preserving legacy SDK exception precedence.
   • Enforces the legacy 100 KB payload size limit and regex task name validation.

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Verification

All changes were deployed and validated against live App Engine Standard (PHP 8.2) in project rshivhare-demo-2016950:

Test Suite Case	Description	Result
Test 1: Push Single Task	Standard push task to default queue	PASS
Test 2: Deduplication	Duplicate task name throws TaskAlreadyExistsException	PASS
Test 3: Push Batch	Multiple tasks enqueued via addTasks	PASS
Test 4: Invalid Task Name	Spaces in task name throws InvalidArgumentException	PASS
Test 5: Task Too Large	Payload > 100 KB throws TaskQueueException	PASS
Test 6: Unknown Queue	Non-existent queue throws TaskQueueException (FAILED_PRECONDITION)	PASS