standard-tests[patch]: use `has_structured_output` property to engage structured output tests (#29635)

Motivation: dedicated structured output features are becoming more
common, such that integrations can support structured output without
supporting tool calling.

Here we make two changes:

1. Update the `has_structured_output` method to default to True if a
model supports tool calling (in addition to defaulting to True if
`with_structured_output` is overridden).
2. Update structured output tests to engage if `has_structured_output`
is True.