fix: elevate timezone warning and implement set_timezone_to_utc() for BigQuery and ClickHouse

[dtsong]

Problem

_connect.py:298-303 silently catches NotImplementedError from set_timezone_to_utc() at DEBUG level and proceeds normally. BigQuery and ClickHouse both raise here.

This is silent data corruption. When comparing timestamps across databases where one session is in a non-UTC timezone, the bisection algorithm produces wrong ranges and phantom diffs — or worse, masks real diffs.

Scope

1. BigQuery (bigquery.py:154): Implement using SET @@time_zone = 'UTC' session variable
2. ClickHouse (clickhouse.py:98): Implement using SET session_timezone = 'UTC'
3. _connect.py:298-303: Elevate the caught NotImplementedError log from DEBUG to WARNING. Consider adding a --require-utc flag that makes this an error.

Key Files

• data_diff/databases/_connect.py:296-304
• data_diff/databases/bigquery.py:154
• data_diff/databases/clickhouse.py:98

Acceptance Criteria

• BigQuery set_timezone_to_utc() sets session to UTC
• ClickHouse set_timezone_to_utc() sets session to UTC
• Connection factory logs WARNING (not DEBUG) when timezone cannot be set
• Tests cover timezone normalization for both databases