Stdlib: set_include_path('') / set_include_path(false) — accepts invalid path, Zend returns false (ext/standard/dir.c) #12165

Summary

set_include_path() with an empty string or false must return false and leave the include path unchanged (Zend/php-src). This compiler accepts the value, returns the previous path string, and mutates get_include_path().

Repro

./script/docker-exec.sh -- bash -lc 'php bin/vm.php test/repro/issue_set_include_path_invalid.php'
./script/docker-exec.sh -- bash -lc 'php test/repro/issue_set_include_path_invalid.php'

Zend (expected) for set_include_path(''):

• return: false
• get_include_path() unchanged

VM (today):

• return: previous path (e.g. '.:/usr/share/php')
• get_include_path() becomes ''

Same pattern for set_include_path(false).

php-src reference

• ext/standard/dir.c — PHP_MINIT_FUNCTION / zif_set_include_path validation of path argument

Implementation (PHP-in-PHP)

• Validate argument in ext/standard/set_include_path.php + VmString/VmFs helper; reject empty/non-string per php-src.
• Register parity in capability matrix if needed.

Done when (php-src-strict)

• set_include_path('') → false, path unchanged
• set_include_path(false) → false, path unchanged
• Valid path still returns previous path string
• Targeted compliance/repro guard

Refs #1492