fix(#633): handle invalid UUIDs in not_found_by parameter

[stefopl]

• fix(Handle invalid UUIDs in found_by parameter within prepare_common_search_params() #633): handle invalid UUIDs in not_found_by parameter
• refactor: extract duplicate found_by and not_found_by filter logic into a reusable method

[mlekorlz]

Tested on both branches:

• OCPL: local install from the devel dump, 56k caches, 2.7M log entries, PHP 8.1.
• OCDE: local oc-server3 (development) on ddev, PHP 8.2, OKAPI pulled by composer at the same
  3aab5717 this branch is based on.

Reproduced #633 for not_found_by on master and confirmed it is fixed here. Same requests against
services/caches/search/all:

parameter	OCPL master	OCPL this PR	OCDE master	OCDE this PR
found_by=<existing uuid>	200 json	200 json	200 json	200 json
found_by=unknown-user	400 json	400 json	400 json	400 json
found_by=00000000-0000-0000-0000-000000000000	400 json	400 json	400 json	400 json
not_found_by=<existing uuid>	200 json	200 json	200 json	200 json
not_found_by=unknown-user	500 text/plain	400 json	500 text/plain	400 json
not_found_by=00000000-0000-0000-0000-000000000000	500 text/plain	400 json	500 text/plain	400 json

The search_and_retrieve path from the issue, the one c:geo actually uses, behaves the same way on
both installs: 500 text/plain on master, 400 json here.

Both USE_SQL_SUBQUERIES paths are covered. OCPL sets it to true; OCDE does not set it at all, so
it takes the Settings.php:237 default of false and runs get_found_cache_ids() instead of the
subquery. I also flipped it to false on OCPL to be sure. Same results everywhere.

No behaviour change on valid input. Responses are byte-identical between master and this branch on
both installs, for found_by, not_found_by, both parameters combined, and with a large limit.

One remark below.

[mlekorlz]

recommended_by, ten lines further down, has the same defect this PR fixes and is not covered.
Same on both branches:

/okapi/services/caches/search/all?recommended_by=00000000-0000-0000-0000-000000000000
OCPL master / this PR:  500 text/plain
OCDE master / this PR:  500 text/plain

SearchAssistant.php:497-507 on master is the same shape as the code being extracted here: same
services/users/users call, same unguarded array_map over the result.

It also throws with the wrong parameter name:

// SearchAssistant.php:503, inside the recommended_by handler
throw new InvalidParam('not_found_by', $e->whats_wrong_about_it);

Folding recommended_by into apply_found_by_filter() is not a straight fit, since it needs
get_recommended_by_sql() rather than the cache_id in (...) form, so it may belong in a separate
PR. The wrong parameter name at :503 is a one-line fix either way.