feat: add handling identity insert

[TimvdHeijden]

Upsert operations failed with IDENTITY_INSERT is not set error when using IDENTITY (auto-increment) columns as id_field.

Solution

• Detects IDENTITY columns via sys.identity_columns query
• Raises descriptive error with actionable alternatives (use method='append', use non-IDENTITY columns, or enable preserve_identity=True)
• New preserve_identity parameter wraps MERGE with SET IDENTITY_INSERT ON/OFF when explicitly enabled
• Logs warning about sequence generation risks

Benefits

• Prevents silent failures, educates users on best practices, maintains backward compatibility.