fix(expressions): wrap string alias in TableAlias in to_table()

[Tigeneric]

Problem

to_table() passes alias as a raw string via table.set(k, v), but qualify_columns expects TableAlias(this=Identifier(...)). This causes qualify_columns to fail on AST-built queries with table aliases.

Repro:

from sqlglot import exp
from sqlglot.optimizer.qualify_columns import qualify_columns

table = exp.to_table("foo", alias="bar")
select = exp.Select().select("*").from_(table)
qualify_columns(select, schema={"foo": {"id": "INT"}})
# TypeError — alias is str, not TableAlias

Fix

Wraps string alias in TableAlias(this=to_identifier(v)) before table.set() — same pattern already used in table_() (line 513).

2-line change, no behavior change for non-string aliases.