fix: address review feedback for fallback selectivity heuristic

Rework compute_fallback_selectivity to resolve all issues raised by
@asolimando:

1. Default selectivity is now applied at most once for all unhandled
   conjuncts combined, instead of multiplied per-conjunct. A predicate
   with three unhandled terms (e.g. LIKE, <>, IN) stays at 0.2, not
   0.2^3 = 0.008.

2. For col_a = col_b equalities, use 1/max(NDV(left), NDV(right))
   instead of only considering the left side's NDV.

3. Restore tests that were accidentally removed during the merge with
   main (fetch_preserves_singleton, fetch_statistics_match_execution,
   fetch_null_column_statistics, singleton_precision).

4. Add unit tests for compute_fallback_selectivity covering:
   - single handled equality (1/NDV)
   - several unhandled conjuncts (default applied once)
   - mixed handled and unhandled (1/NDV * default)
   - col = col with max(NDV)
   - non-equality predicate (default only)
   - CAST(col) = expr (not handled, shows current behavior)
   - utf8 col = literal (handled via NDV)

5. Add SLT plan test with scalar subquery equality showing the new
   row estimate.

Note: utf8_col = 'literal' also fails check_support, so it now gets
1/NDV instead of 20%. This is generally more accurate since a Utf8
column with 60 distinct values should estimate ~1.7% selectivity for
an equality match, not a blanket 20%.

Author: mohitgurav20