feat: dbt-core 1.12 compatibility, Python 3.14 support, and transaction-safety fixes

[axellpadilla]

Summary

Official support for dbt-core 1.12 and Python 3.14, plus two behavior-flag
defaults flipped to their new (non-legacy) values, plus the fixes those
flips exposed.

dbt-core 1.12 / Python 3.14

• Bump dbt-core minimum to >=1.12.0, dbt-adapters to >=1.24.5.
• Add Python 3.14 to classifiers and the CI/publish-docker matrices.
• Drop the pathspec override in pyproject.toml (no longer needed on 1.12).
• Add a deprecation warning for the vars.dbt_sqlserver_use_default_schema_concat
  fallback in schema.sql (use flags.* instead).

Behavior changes (both now default True; legacy False is deprecated)

• dbt_sqlserver_use_dbt_transactions: dbt-managed transaction hooks now emit
  real BEGIN TRANSACTION / COMMIT TRANSACTION instead of no-ops, so a
  failed model rolls back its own statements instead of leaving a partial
  result on autocommit.
• dbt_sqlserver_use_native_string_types: STRING → VARCHAR(MAX), NCHAR →
  NCHAR(1), NVARCHAR → NVARCHAR(4000), replacing the legacy
  VARCHAR(8000)/CHAR(1) mappings as the default.

Bugfixes: making the transactions flip actually safe

Turning transactions on by default means a model's whole build now runs
inside one continuous ambient SQL Server transaction (first statement through
its own trailing adapter.commit()). Two places still assumed the old
autocommit-per-statement behavior and broke:

• Creating an index with build_options: {online: true} or {resumable: true}
  on a fresh table, or on the rename-swap path — sqlserver__create_indexes
  never split these out of the transaction, so it hit SQL Server error 574
  (RESUMABLE cannot run inside a user transaction). Index reconciliation
  already had a creates_no_txn split, but run_query's auto_begin=false
  only skips starting a new transaction — it doesn't escape one already open.
• The dbt_full_refresh_incomplete crash marker (full_refresh_build=prebuilt
  and incremental full refreshes) shared the same ambient transaction as the
  rebuild it exists to survive, so a real failure mid-rebuild rolled the
  marker back along with everything else — defeating its purpose.

Fixed with two new adapter primitives, commit_if_open() / begin_if_closed(),
used to flush the ambient transaction around both cases: a shared
sqlserver__create_indexes_no_txn helper for the index case, and a split of
sqlserver__create_table_as_prebuilt into two real statements (setup+marker,
then load) for the marker case.

Test infra

• Local functional tests now default to the mssql-python backend
  (SQLSERVER_TEST_BACKEND still overrides; CI always sets this explicitly
  per matrix cell, so it's unaffected).
• Removed the dbt_sqlserver_use_dbt_transactions: False class-level
  opt-outs from test_index_config.py / test_full_refresh_build.py — an
  intermediate stop-gap now made unnecessary by the bugfix above.
• test_native_string_types.py / test_transactions.py updated for the new
  defaults (default case now asserts native types; legacy case renamed and
  asserts the deprecated mappings under the flag).

Test plan

• Full tests/functional/adapter/mssql suite (135 passed, 2 skipped) and
  tests/unit (312 passed) under pyodbc.
• test_index_config.py, test_full_refresh_build.py, test_transactions.py,
  test_xact_abort.py, tests/unit re-verified under mssql-python
  (365 passed, 1 xfailed).
• Reproduced the two transaction bugs live against SQL Server before
  fixing (SQL error 574 on RESUMABLE; marker lost after a real mid-rebuild
  failure), confirmed both fixed after.

Closes #764, closes #662