fix: remove unreachable created_at backfill in update_task_status

[bsbodden]

Summary

• Remove dead backfill code in update_task_status that checked if task.created_at is None
• Task.created_at is a non-optional datetime field with default_factory, so model_validate_json rejects null values before the backfill could execute
• Remove unused UTC import

Changes

• agent_memory_server/tasks.py: Remove unreachable lines 95-97, remove unused UTC import
• tests/integration/test_task_created_at_invariant.py: 3 tests documenting the created_at invariant and confirming null values in Redis are treated as corrupt

Test plan

• All 3 new integration tests pass
• Full existing test suite passes (710 passed, 0 failures)

Closes #208

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Note

Low Risk
Low risk: removes unreachable created_at backfill logic and adds integration coverage; runtime behavior only changes in that update_task_status no longer mutates tasks on the (already-unparseable) corrupt created_at=null case.

Overview
Removes the unreachable created_at backfill in update_task_status (and the now-unused UTC import), relying on the Task model’s non-null created_at invariant rather than attempting to patch invalid data at update time.

Adds an integration regression suite asserting created_at is set on create_task, preserved across status updates, and that tasks with created_at=null stored in Redis are treated as corrupt (i.e., get_task returns None and update_task_status becomes a no-op).

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[github-actions]

Review completed. No suggestions at this time.

[jit-ci]

🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

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^{Security scan by Jit}

[Copilot]

Pull request overview

Removes unreachable created_at backfill logic from task status updates and adds regression coverage to enforce the invariant that Task.created_at is always non-null (and that created_at: null data in Redis is treated as corrupt/unrecoverable).

Changes:

• Removed dead created_at is None backfill in update_task_status (and dropped the now-unused UTC import).
• Added integration tests asserting created_at is set on create, preserved on updates, and that created_at=null payloads in Redis cannot be parsed/updated.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
agent_memory_server/tasks.py	Deletes unreachable created_at backfill and removes unused UTC import while preserving datetime for type hints.
tests/integration/test_task_created_at_invariant.py	Adds regression tests that document/enforce the created_at non-null invariant and corrupt-data handling.

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