fix(resolver): correctly resolve unnest alias shadowing for BigQuery

[fivetran-MichaelLee]

When BigQuery SQL uses an UNNEST expression where the table alias matches a column name in another table, SQLGlot's qualify optimizer step fails due to perceived ambiguous column name.

To illustrate, this is valid SQL in BigQuery that fails to qualify:

WITH temp_data AS (
  SELECT DATE('2024-01-01') as timeline_date, 100 as val
)
SELECT 
  timeline_date  -- sqlgot can't resolve
FROM 
  UNNEST(GENERATE_DATE_ARRAY('2024-01-01', '2024-01-02', INTERVAL 1 DAY)) AS timeline_date
LEFT JOIN 
  temp_data ON timeline_date = temp_data.timeline_date;

To fix, resolver is updated as follows:

• get_table will check alias.column[0] for the actual alias when UNNEST_COLUMN_ONLY is true and alias.this is None
• _get_unambiguous_columns will check ambiguous columns against aliases in unnest sources

[github-actions]

SQLGlot Integration Test Results

Comparing:

• this branch (sqlglot:mlee/qualify-error, sqlglot version: mlee/qualify-error)
• baseline (main, sqlglot version: 28.5.1.dev77)

⚠️ Limited to dialects: snowflake, duckdb

By Dialect

dialect	main	sqlglot:mlee/qualify-error	difference	links
duckdb -> duckdb	4003/4003 passed (100.0%)	4003/4003 passed (100.0%)	No change	full result / delta
snowflake -> duckdb	579/847 passed (68.4%)	579/847 passed (68.4%)	No change	full result / delta
snowflake -> snowflake	847/847 passed (100.0%)	847/847 passed (100.0%)	No change	full result / delta

Overall

main: 5697 total, 5429 passed (pass rate: 95.3%), sqlglot version: 28.5.1.dev77

sqlglot:mlee/qualify-error: 5697 total, 5429 passed (pass rate: 95.3%), sqlglot version: mlee/qualify-error

Difference: No change