fix(sql_database): reflect timestamp/datetime precision at full_with_precision

[Aryan-Pardeshi]

Fixes #4316

Problem

sqla_col_to_column_schema never reads .precision from DateTime types, so timestamp/datetime columns lose their fractional-seconds precision even at reflection_level="full_with_precision" (the level explicitly gated by add_precision). Only Numeric, SmallInteger, String, and _Binary had precision handling.

Change

In the DateTime branch, when add_precision is set and the reflected type exposes a precision attribute (e.g. Oracle TIMESTAMP(p), MSSQL DATETIME2(p)), emit col["precision"]. Guarded with getattr since generic sa.DateTime has no such attribute.

Tests

• test_timestamp_precision_reflected_full_with_precision — DATETIME2(3) → precision: 3 at full_with_precision
• test_timestamp_precision_omitted_without_full_with_precision — no precision key at "full"
• test_generic_datetime_has_no_precision_key — generic DateTime emits no precision key

Verified: tests/sources/sql_database/test_schema_types.py — 20 passed under SQLAlchemy 2.0.51 and 2.1.0b3.