fix(redshift): support APPROXIMATE PERCENTILE_DISC round-trip

[fivetran-kwoodbeck]

APPROXIMATE PERCENTILE_DISC(p) WITHIN GROUP (ORDER BY x) is valid Redshift syntax that was causing a ParseError on round-trip. _parse_approximate_count only handled APPROXIMATE COUNT(DISTINCT ...) — when it saw a PercentileDisc it retreated, leaving APPROXIMATE stranded in the projection list.

To fix, another branch was added to _parse_approximate_count and generator was updated to output exp.ApproxQuantile correctly.

[github-actions]

SQLGlot Integration Test Results

Comparing:

• this branch (sqlglot:fix/redshift-approximate-percentile, sqlglot version: fix/redshift-approximate-percentile)
• baseline (main, sqlglot version: 0.0.1.dev1)

By Dialect

dialect	main	sqlglot:fix/redshift-approximate-percentile	transitions	links
bigquery -> bigquery	24647/24652 passed (100.0%)	23497/23497 passed (100.0%)	No change	full result / delta
bigquery -> duckdb	867/1154 passed (75.1%)	0/0 passed (0.0%)	Results not found	full result / delta
duckdb -> duckdb	5823/5823 passed (100.0%)	0/0 passed (0.0%)	Results not found	full result / delta
snowflake -> duckdb	1063/1961 passed (54.2%)	0/0 passed (0.0%)	Results not found	full result / delta
snowflake -> snowflake	65133/65133 passed (100.0%)	63027/63027 passed (100.0%)	No change	full result / delta
databricks -> databricks	1370/1370 passed (100.0%)	1370/1370 passed (100.0%)	No change	full result / delta
postgres -> postgres	6042/6042 passed (100.0%)	6042/6042 passed (100.0%)	No change	full result / delta
redshift -> redshift	7101/7101 passed (100.0%)	7101/7101 passed (100.0%)	No change	full result / delta

Overall

main: 113236 total, 112046 passed (pass rate: 98.9%), sqlglot version: 0.0.1.dev1

sqlglot:fix/redshift-approximate-percentile: 101037 total, 101037 passed (pass rate: 100.0%), sqlglot version: fix/redshift-approximate-percentile

Transitions:
No change

Dialect pair changes: 0 previous results not found, 3 current results not found

✅ 38 test(s) passed