Support Datetime Feature +/- pd.Timedelta (Issue #629)

[The-Fallen-Phoenix]

• Update AddNumericScalar to accept Datetime input types with Timedelta values
• Update SubtractNumericScalar to accept Datetime input types with Timedelta values
• Add comprehensive tests for datetime + timedelta and datetime - timedelta operations
• Both operations now properly validate input types and return datetime results

Fixes #629

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