fix(snowflake): Fix annotate_types crashes for FLATTEN over STRUCT and WithinGroup

[georgesittas]

• Fix _annotate_by_args to propagate UNKNOWN type upwards when any arg is UNKNOWN, instead of silently coercing to a known type (regression from chore: _annotate_by_args UNKNOWN #6703)
• Fix _annotate_within_group to fall back to the inner function's type for non-percentile aggregates like ARRAY_AGG (regression from feat(snowflake)!: annotate types for PERCENTILE_DISC and WithinGroup #6300)

[github-actions]

SQLGlot Integration Test Results

Comparing:

• this branch (sqlglot:jo/fix_annotate_types_issues, sqlglot version: jo/fix_annotate_types_issues)
• baseline (main, sqlglot version: 0.0.1.dev1)

By Dialect

dialect	main	sqlglot:jo/fix_annotate_types_issues	transitions	links
bigquery -> bigquery	23872/23877 passed (100.0%)	21252/21252 passed (100.0%)	No change	full result / delta
bigquery -> duckdb	2106/2624 passed (80.3%)	0/0 passed (0.0%)	No change	full result / delta
snowflake -> duckdb	1512/2674 passed (56.5%)	1512/2674 passed (56.5%)	No change	full result / delta
snowflake -> snowflake	65910/65910 passed (100.0%)	65910/65910 passed (100.0%)	No change	full result / delta
databricks -> databricks	1364/1364 passed (100.0%)	1364/1364 passed (100.0%)	No change	full result / delta
postgres -> postgres	6040/6040 passed (100.0%)	6040/6040 passed (100.0%)	No change	full result / delta
redshift -> redshift	7059/7059 passed (100.0%)	7059/7059 passed (100.0%)	No change	full result / delta

Overall

main: 109548 total, 107863 passed (pass rate: 98.5%), sqlglot version: 0.0.1.dev1

sqlglot:jo/fix_annotate_types_issues: 104299 total, 103137 passed (pass rate: 98.9%), sqlglot version: jo/fix_annotate_types_issues

Transitions:
No change

✅ 4 test(s) passed