Add edge case tests to improve C++ code coverage

Add test_021_coverage_edge_cases.py with 10 validated tests targeting uncovered C++ code paths:

- DECIMAL(38,18) with maximum precision

- TIME(7) and TIME(0) for SQL_SS_TIME2 paths

- DATETIMEOFFSET with extreme timezones (±14:00) and year boundaries

- DATE standalone type boundaries

- UNIQUEIDENTIFIER edge cases (all-zeros, all-Fs, nil UUID)

- VARBINARY with empty, small, and 16KB (LOB path) values

- BINARY(10) fixed-length padding behavior

- Combined multi-type result sets

These tests target ddbc_bindings.cpp gaps identified in coverage analysis.

All tests pass locally and are based on proven patterns from existing tests.

Target: Increase coverage from 81% to 85%+

Author: gargsaumya