fix(tsql): ALTER COLUMN with a nullability clause is parsed as a Command

[sravankumarkunadi]

Description

ALTER TABLE t ALTER COLUMN c INT NOT NULL doesn't parse in T-SQL and falls back to a Command. Macros aren't resolved inside a Command, so @SQL('@this_model') reaches SQL Server verbatim and fails with Incorrect syntax near '@SQL'.

sqlglot's _parse_alter_table_alter returns as soon as it has the type, so T-SQL's trailing NOT NULL is left unconsumed and parsing raises. The ADD CONSTRAINT variant in the issue works because it parses cleanly.

Adds a T-SQL-scoped parser override that consumes the trailing NULL / NOT NULL into allow_null, plus a generator override to emit it — sqlglot's altercolumn_sql returns at the dtype branch before it reaches allow_null. Both only engage when a dtype is present, so SET NOT NULL / DROP NOT NULL keep their existing path.

Post-statement from the issue, creating stage:

- ALTER TABLE @SQL('@this_model') ALTER COLUMN id INT NOT NULL   # + "could not be semantically understood" warning
+ ALTER TABLE [cat].[sqlmesh__test].[test__demo__3571846471] ALTER COLUMN [id] INTEGER NOT NULL

Doesn't address the reporter's aside about declaring a column NOT NULL without a post-statement.

Closes #5932

Test Plan

• test_tsql_alter_column_nullability (test_dialect.py) — parse/round-trip for INT NOT NULL, INT NULL, bare INT, the @IF form from the issue, and DROP NOT NULL. Fails on main.
• test_tsql_alter_column_post_statement (test_model.py) — render_post_statements resolves @this_model at the creating stage.
• pytest tests/core/test_dialect.py tests/core/test_model.py tests/core/test_macros.py → 609 passed vs 607 on main; both runs hit the same 26 pre-existing sushi_context fixture errors (Materialization strategy ... was not found).

Checklist

• I have run make style and fixed any issues — ruff format/ruff check/mypy on the changed files, not pre-commit run --all-files
• I have added tests for my changes (if applicable)
• All existing tests pass (make fast-test) — ran the three files above, see the pre-existing local errors noted there
• My commits are signed off (git commit -s) per the DCO