fix: apply session-level SET XACT_ABORT ON to all connections

[axellpadilla]

Summary

Closes #718.

table_dml_refresh and other multi-statement paths (index reconcile, prebuilt full-refresh) send DML as one autocommit batch (BEGIN; DELETE; INSERT; COMMIT). A statement-abort error (NOT NULL, constraint violation, etc.) only aborts that statement, not the batch — execution falls through to COMMIT, silently persisting the DELETE on an empty target even though dbt reports failure. This also happens with no explicit transaction at all, so the fix can't depend on dbt_sqlserver_use_dbt_transactions.

Fix: SET XACT_ABORT ON as a session-level default on every connection, so a run-time error kills the whole batch and rolls back instead of falling through to COMMIT.

Changes

• xact_abort: bool = True credentials field (in _connection_keys(), shows in dbt debug). Independent of dbt_sqlserver_use_dbt_transactions — that flag decides who owns the transaction boundary; xact_abort decides how the server reacts to a mid-batch error, which matters most exactly when there's no explicit transaction.
• open() applies it via new _apply_session_settings(); a connection that fails this setup is closed rather than handed back to dbt.
• Once-per-process warning when disabled, noting the current dbt_sqlserver_use_dbt_transactions state.
• Removed the now-redundant local SET XACT_ABORT ON from indexes.sql and create.sql — session-level is the single source of truth. Comment added above table_dml_refresh pointing at Evaluate SET XACT_ABORT ON for SQL Server DML refresh transactions #718.
• README + CHANGELOG (labeled as a behavior change, default-on in an already-published 1.11.x line).

Testing

New functional suite (test_xact_abort.py) against a real SQL Server:

• Seed 2 rows via DML refresh, second run yields NULL → run fails and the 2 rows survive (the row-count assertion is the point; pre-fix, the run also fails but the table is empty).
• xact_abort=True × dbt_sqlserver_use_dbt_transactions on/off: rows preserved both ways.
• xact_abort=False + transactions on: preserved via dbt's own rollback, not XACT_ABORT.
• xact_abort=False + transactions off (both disabled): not asserted as passing — manually confirmed this still loses rows, which is the documented, intentional limit of disabling the flag, not a regression.
• @@OPTIONS & 16384 bit check: 1 by default, 0 when disabled.

Full suite: pytest tests/unit tests/functional -n auto → 600 passed, 48 skipped, 2 xfailed, 0 failed (298s).

Also fixed along the way: a pre-existing test-isolation bug where unit tests stub a process-global driver-module cache (_RUNTIME_STATE) without resetting it, leaking a fake module into real functional tests when both run in one pytest process. Added an autouse reset fixture. Also added -n auto to the integration-tests CI workflow (verified safe locally).

Notes

Based directly on current upstream master, independent of #763 (UDF function resource support) — no shared files besides table_dml_refresh.sql, where this PR's change applies cleanly against the Dynamic Data Masking code already on master.