BUG #19704: ispell dictionary accepts trailing junk in numeric COMPOUNDFLAG

The following bug has been logged on the website:

Bug reference: 19704
Logged by: Qifan Liu
Email address: imchifan(at)163(dot)com
PostgreSQL version: 18.6
Operating system: Linux/amd64
Description:

Creating an ispell text-search dictionary from an affix file containing
COMPOUNDFLAG 1x and FLAG num succeeds. PostgreSQL interprets the malformed
compound flag as numeric flag 1. This can cause subtle text-search behavior
in dictionaries installed from malformed affix files.

Steps to reproduce
------------------
Run as an operating-system user allowed to create files in PostgreSQL's
tsearch_data directory.

set -eu
sd=$(pg_config --sharedir)/tsearch_data
n=pg_numeric_compoundflag
printf 'SET UTF-8\nCOMPOUNDFLAG 1x\nFLAG num\n' > "$sd/$n.affix"
printf '2\nfoo/1\nbar/1\n' > "$sd/$n.dict"
psql -X -v ON_ERROR_STOP=1 -At postgres <<SQL
CREATE TEXT SEARCH DICTIONARY $n (
TEMPLATE = ispell,
DictFile = $n,
AffFile = $n
);
SELECT coalesce(array_to_string(ts_lexize('$n', 'foo'), ','), 'NULL');
SQL

Actual result
-------------
Dictionary creation succeeds, and the malformed COMPOUNDFLAG value is
treated as flag 1:

CREATE TEXT SEARCH DICTIONARY
foo

Expected result
---------------
Dictionary creation should fail with a configuration-file error because "1x"
is not a valid numeric flag. PostgreSQL should not accept the prefix "1",
discard the trailing "x", and subsequently lexize the entry as flag 1.

Additional information
----------------------
The issue was reproduced on PostgreSQL 20devel, PostgreSQL 18.6, and
PostgreSQL 17.11.
Inference: numeric flag conversion validates the converted prefix or range
but does not verify that conversion consumed the entire token.