Fix(scope)!: include bigquery unnest aliases in selected sources

[georgesittas]

This addresses the issue reported in this Slack thread: https://tobiko-data.slack.com/archives/C0448SFS3PF/p1750862162339109. This should only impact users that have queries with UNNEST sources and who build scopes without first calling qualify_tables or qualify on the ASTs.

This example reproduces the error:

from sqlglot import parse_one
from sqlglot.optimizer.scope import build_scope

query = """
SELECT
    ID
    , release
    FROM table
    LEFT JOIN UNNEST(environments) AS env
    LEFT JOIN UNNEST(env.deploySteps) AS release
"""

parsed = parse_one(query, dialect="bigquery")
scope = build_scope(parsed)

print(scope.selected_sources)