pgsql: Avoid overflow in Levenshtein distance calculations.

Avoid overflow in Levenshtein distance calculations.

levenshtein() and levenshtein_less_equal() let the caller specify
the insertion, deletion, and substitution costs, and
fuzzystrmatch's corresponding SQL functions accept any 32-bit
integer for each.  Since the distances are calculated with 32-bit
arithmetic, large costs can cause overflows, thereby producing
nonsensical results.  Certain inputs to levenshtein_less_equal()
can even cause out-of-bounds writes.  To fix, use 64-bit arithmetic
instead, and error whenever the final result won't fit in the
returned 32-bit integer.

We may want to teach these functions to reject negative costs, too,
but that didn't seem appropriate for a security fix, and therefore
it is left as a future exercise.

Reported-by: Ben Morris in collaboration with Claude and Anthropic Research
Author: Nathan Bossart 
Reviewed-by: Dean Rasheed 
Security: CVE-2026-15742
Backpatch-through: 14

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/74916136fc6c9fcc5a94e1b53863703593009583
Author: Nathan Bossart 

Modified Files
--------------
contrib/fuzzystrmatch/expected/fuzzystrmatch.out | 14 ++++
contrib/fuzzystrmatch/sql/fuzzystrmatch.sql      |  3 +
src/backend/utils/adt/levenshtein.c              | 89 ++++++++++++------------
src/backend/utils/adt/varlena.c                  | 14 ++++
4 files changed, 77 insertions(+), 43 deletions(-)