chore: prepare 1.12.0rc3 release

[axellpadilla]

Summary

• bump the adapter version to 1.12.0rc3
• align README and release documentation with dbt-core 1.12, Python 3.14, native string mappings, and dbt-managed transactions enabled by default
• document the OpenQuery feature and identifier-quoting fix
• fix delimiter-schema functional test fixtures for dbt-core schema concatenation

Validation

• 500 unit tests passed
• 24 affected SQL Server functional tests passed
• Ruff, pre-commit hooks, and diff checks passed