Fix SQL Server defaults when modifying columns

[elpete]

Closes #152

What changed

• remove an existing SQL Server default constraint before altering a column with a new default
• discover the existing constraint through SQL Server system catalogs so custom constraint names are supported
• add the requested default back as a valid standalone constraint after the column alteration
• add a regression test using the reproduction from the issue

Root cause

The SQL Server modify-column compiler reused the create-column compiler, which placed CONSTRAINT ... DEFAULT inline in an ALTER COLUMN statement. SQL Server does not accept default constraints in that position.

Validation

• TestBox on Lucee 6: 2,773 passed, 0 failed, 3 skipped
• box run-script format:check
• git diff --check

[elpete]

Updated the implementation to address the dynamic-SQL injection concern:

• the EXEC statement no longer interpolates caller-provided table names
• schema and table names are resolved from SQL Server catalog metadata and passed through QUOTENAME
• SQL Server identifier wrapping now escapes closing brackets as ]]
• added an adversarial regression test covering apostrophes and closing brackets in table and column names

Validation: Lucee suite 2,915 passed, 0 failed/errors, 3 skipped; formatting and git diff --check pass.