Return a string from the mocked recursiveTagParse

[malberts]

Why

MediaWiki master declares Parser::recursiveTagParse() and recursiveTagParseFully()
with a string return type (the parameter stays untyped). The component tests mock these
with returnArgument(0), echoing the input verbatim. When a component parses an attribute
value that is a bool or int, the mock returns that non-string and PHPUnit's typed mock
throws:

TypeError: Mock_Parser::recursiveTagParse(): Return value must be of type string, true returned

Why an attribute value is a bool

This is not an unrealistic fixture. ParserRequest builds its attribute array from what the
parser passes in, and for the parser-function syntax a valueless option is stored as a PHP
true (an empty one as false) by ParserRequest::getKeyValuePairFrom(). So
[ 'disabled' => true ] is exactly what {{#bootstrap_button:text|disabled}} produces; the
tag-extension syntax (<bootstrap_button disabled>) yields a string instead. Both reach
recursiveTagParse().

In production the real recursiveTagParse() coerces such a value and returns a string, so it
works. The mock's returnArgument(0) did not coerce, which is the only reason the typed
return now fails.

What

Make the recursiveTagParse / recursiveTagParseFully mocks return (string)$text, so the
mock behaves like the real parser and honours the string return contract. The realistic
bool/int inputs are kept and no assertions change (true stays truthy as "1", 0 stays
falsy as "0"). The renderErrorMessage mock is left as-is.

Verification

Test-only change. On MediaWiki master the component tests that hit the TypeError now pass;
the older rows are unaffected, since the cast is a no-op for the string inputs they use.