Fix missing input and output modality combinations, fixing usage of Nano Banana (among other problems)

[felixarntz]

Fixes #10

Great catch @dkotter! This pointed me to other similar omissions (which were probably less critical, but still could cause expected features to not work).

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Co-authored-by: felixarntz <flixos90@git.wordpress.org>
Co-authored-by: gziolo <gziolo@git.wordpress.org>
Co-authored-by: dkotter <dkotter@git.wordpress.org>

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[gziolo]

Overall, this looks good. I noticed a nearly identical fix in WordPress/ai-provider-for-openai#11.

I assume it's something every provider might need, so having a utility that simplifies configuration could help. I was thinking about using binary representation for optional modalities (not tested, but it should illustrate the idea):

/**
 * Build all modality combinations that always include the required modalities
 * with every possible subset of the optional modalities.
 *
 * @param array $required Required modalities included in every combination.
 * @param array $optional Optional modalities to generate subsets from.
 * @return array List of modality combinations.
 */
function build_modality_combinations( $required, $optional ) {
    $combinations = array();
    $count         = count( $optional );

    for ( $i = 0; $i < ( 1 << $count ); $i++ ) {
        $combo = $required;

        for ( $j = 0; $j < $count; $j++ ) {
            if ( $i & ( 1 << $j ) ) {
                $combo[] = $optional[ $j ];
            }
        }

        $combinations[] = $combo;
    }

    return $combinations;
}

Example usage:

$allModalityCombinationsWithText = build_modality_combinations(
    array( ModalityEnum::text() ),
    array( ModalityEnum::image(), ModalityEnum::audio(), ModalityEnum::document() )
);
$allModalityCombinationsTextAudio = build_modality_combinations(
    array(),
    array( ModalityEnum::text(), ModalityEnum::audio() )
);

[felixarntz]

@gziolo Thank you! I opened WordPress/php-ai-client#215 to introduce such a helper, great idea. For now I'm going to proceed as is, and we can revise this once the helper has been released.