[belisoful]

Part of #1497. Two commits, to be reviewed separately.

1. Unions: a value no member can hold passes through unchanged

TPropertyValue::coerceToType(['a'], int|string) raised TPhpErrorException: Array to string conversion, and an object without __toString() raised PHP's \Error. Step 4 of _coerceUnionType() returned an array only for a union with an array member and otherwise fell through to a string cast.

Such a value is now returned unchanged, the miss style _coerceToClass() already documents, so a typed setter raises PHP's own TypeError ("must be of type string|int, array given") and the property stays unassigned. array|string and iterable|string keep passing an array through, and nullable unions, ICoercible, enums, Stringable objects, and TJavaScriptLiteral behave as before. Two cases that threw now succeed because the setter accepts the value: an object to object|int, and a callable array to callable|string.

2. Single string: ensureString() throws TInvalidDataTypeException

The same defect existed for string and ?string through ensureString(), whose (string) cast raised the warning, or returned 'Array' when warnings were masked.

• coerceToType() to string or ?string passes an array or an object without __toString() through, as in commit 1.
• ensureString() called directly throws TInvalidDataTypeException with the new propertyvalue_invalid_string_type, "Value of type '{0}' cannot be converted to a string." Most setters are untyped and store ensureString($value), so pass-through there would store the array. It throws the same way with warnings reported or masked.
• Everything else ensureString() does is unchanged: bool, NAN, null, numbers, Stringable objects, TJavaScriptLiteral.

Behavior to review:

• With warnings masked, ensureString($array) returned 'Array'; it now throws.
• TException::__construct() converts each message argument with ensureString(). new TFooException('code', $array) throws TInvalidDataTypeException; with warnings reported it already threw TPhpErrorException. Rendering such an argument as its type is a possible follow-up.
• No call in framework/ passes an array or such an object to ensureString() on purpose, and no existing test does. Sites where input can be an array, such as a POST field field[] in TWebTemplate::loadPostData(), now get the clear exception where they got the warning exception.

Regression tests for both commits; they pass on PHP 8.2 through 8.5.