Fix differential CI with ClickHouse 26.9

[ShawnChen-Sirius]

Summary

• avoid ClickHouse 26.9 parsing INSERT ... VALUES seed data as TSV
• append SQL-native FORMAT to the final query for output formatting
• add a regression test for the differential runner

Validation

• ruff and mypy pass
• 315 unit/contract tests pass
• 25 differential tests pass against ClickHouse 26.5
• 24/24 differential scenarios pass against ClickHouse 26.9.2

Fixes the scheduled differential CI failure.

Note

Fix differential CI with ClickHouse 26.9 by moving output format to SQL

The ClickHouse 26.9 release makes the CLI --format option also control INSERT input parsing, which breaks multi-statement SQL blocks with inline VALUES. ClickHouseLocal.run in conftest.py now trims trailing semicolons and appends a SQL-level FORMAT clause instead of passing --format to the subprocess. Adds a subprocess-mocked test in test_differential_runner.py that asserts the appended FORMAT TabSeparated and the removed CLI flag.

^{Macroscope summarized 41680fe.}