async getBySourceAndTarget(source, target) {
        const model = await this.#MentionModel.findOne({
            source: source.href,
            target: target.href
        }, {require: false});

        if (!model) {
            return null;
        }

        return this.#modelToMention(model);
    }
@param {URL} source
@param {URL} target
@returns {Promise<import('./Mention')|null>}