Reservation support

[0xRobin]

No description provided.

[cursor]

PR Summary

Medium Risk
Changes how every query text is constructed/executed when reservation is set (multi-statement SQL), which could affect compatibility with clients/queries that don’t support scripting or rely on exact SQL text.

Overview
Adds DSN-level reservation support by parsing reservation (or legacy reservation_id) from the connection string and storing it in driver config.

When configured, the driver now transparently prefixes executed SQL with a SET @@reservation='...' statement (escaping quotes and avoiding double-prefixing) so queries run under the specified reservation; tests were updated to allow overriding the test DSN via BIGQUERY_TEST_DSN, and README documents the new DSN option.

^{Written by Cursor Bugbot for commit fe532d3. Configure here.}