[6.x] Match contains_any values literally in field conditions

[joshdaugherty]

Fixes #15530

contains_any built a regular expression from its values without escaping them, so a.b matched axb, (555) matched any value containing 555, $100 never matched, and C++ threw SyntaxError: Invalid regular expression.

This matches each value as a literal substring instead, which keeps the operator's substring semantics (contains_any foo still matches foobar) without reading any value as a pattern. A null or undefined field value is now treated as an empty string. Before, RegExp.prototype.test coerced it to the text "null" or "undefined", so contains_any ul matched an empty field. The array branch is unchanged: it already compares values exactly through intersection.

includes_any is not touched; #13135 already gave it exact matching.

Tests

Adds it matches contains_any values literally rather than as patterns to resources/js/tests/FieldConditionsValidator.test.js, covering ., (…), + and $. It fails on 6.x without the change (axb matches a.b) and passes with it; the file's other 52 tests still pass, and npx vitest run --project unit passes (628 tests).