• Preserve original memory creation time in created_at field
• Record deletion time in updated_at field
• Apply fix to both sync and async versions
• Add comprehensive test coverage for timestamp preservation

Fixes #4467

Description

This PR implements enhancement #4467 by adding proper timestamp handling for DELETE operations in memory history.

Problem: When memories are deleted, the original creation timestamp is lost because created_at is overwritten with deletion time, making historical analysis impossible.

Solution:

• Preserve original memory creation time in created_at field
• Record deletion time in updated_at field
• Apply fix to both sync (_delete_memory) and async (_adelete_memory) versions

Impact:

• Enables accurate historical analysis of deleted memories
• Maintains temporal integrity of memory lifecycle
• Allows tracking "how long did this memory exist before deletion"

Fixes #4467

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (does not change functionality, e.g. code style improvements, linting)
• Documentation update

How Has This Been Tested?

Tested with the following scenario:

• Environment: Mem0 v1.0.7, Python 3.13, Qdrant (local)
• Test scenarios:
  1. Create memory → verify created_at is set correctly
  2. Delete memory → verify created_at is preserved, updated_at is updated to deletion time
  3. Query history → verify both timestamps are correct and different

Example test output:

# After creation
created_at: 2026-03-20 10:00:00
updated_at: 2026-03-20 10:00:00

# After deletion
created_at: 2026-03-20 10:00:00  # ✓ Preserved
updated_at: 2026-03-21 15:30:00  # ✓ Updated to deletion time

• Unit Test (comprehensive test coverage added)
• Test Script (manual verification performed)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

Maintainer Checklist

• closes Enhancement: Add Timestamp for DELETE Operations in History #4467
• Made sure Checks passed