feat: add unused-asyncio-task error code for asyncio.create_task without reference (#14710)

[DawnofGenX]

Summary

Adds a new opt-in error code unused-asyncio-task that warns when asyncio.create_task() or asyncio.ensure_future() is called without holding a reference to the returned task. This is a known Python gotcha where the task can be garbage collected before it completes.

Changes

• mypy/errorcodes.py: Added UNUSED_ASYNCIO_TASK error code (disabled by default)
• mypy/checker.py: Added check in type_requires_usage() for asyncio.create_task and asyncio.ensure_future calls
• test-data/unit/check-async-await.test: Added two test cases covering both error and no-error scenarios

Test Results

• testAsyncioCreateTaskWithoutReference: PASSED
• testAsyncioCreateTaskWithReference: PASSED
• All 79 async tests: PASSED

Fixes #14710

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅