fix(config): parse default_test_connection from yaml

[manan28025]

Description

Fixes #5584.

default_test_connection in config.yaml fails to load, even with the example from the docs. The connection validator is registered for default_test_connection, but on Config that's only the alias. The field is default_test_connection_. field_validator matches field names, and check_fields=False hides the mismatch, so the validator never ran. The YAML mapping was then validated against the abstract ConnectionConfig and rejected. Python configs weren't affected because they pass an instance.

The fix points the validator at the field name. No other config model has a validator pointed at an alias.

Test Plan

• New test_default_test_connection_from_yaml fails on main with the error from the issue and passes with the fix.
• Ran test_config, test_connection_config, test_test and test_context. The only failures need drivers I don't have installed (pyodbc, PyAthena, PySpark) and fail the same way on main.
• Ran the config from the issue with sqlmesh info and sqlmesh test. It loads and the tests run against test.db.

Checklist

• I have run make style and fixed any issues (ruff and ruff-format pass; mypy only reports pyspark not installed)
• I have added tests for my changes (if applicable)
• All existing tests pass (make fast-test) (ran the related suites above)
• My commits are signed off (git commit -s) per the DCO