fix: preserve omitted offsets in date-property TTL updates

[HuaTNA]

Fixes #2170.

Keep ttl_offset=None unchanged in Reconfigure.ObjectTTL.delete_by_date_property, so partial updates preserve the existing offset. Previously, changing only filter_expired_objects also reset defaultTtl to zero and changed expiry timing. An explicit zero still resets the offset; the creation default remains zero.

Added tests for preserving positive/negative offsets, explicit zero and timedelta values, and the creation default. The two preservation cases fail before the fix. Afterward, test/collection/test_config_update.py and test/collection/test_config.py pass all 233 tests. Ruff lint/format, Flake8, and whitespace checks passed. No live-server integration test was run.

AI assistance was used to investigate, implement, and test this patch.

[orca-security-eu]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Infrastructure as Code	0   0   0   0	View in Orca
Passed	SAST	0   0   0   0	View in Orca
Passed	Secrets	0   0   0   0	View in Orca
Passed	Vulnerabilities	0   0   0   0	View in Orca

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