Final answer:
The most likely diagnosis based on the symptoms and radiographic findings is internal resorption, and the recommended treatment would be root canal therapy.
Step-by-step explanation:
The patient in question complains of thermal sensitivity in her 'front teeth' and exhibits a radiolucent expansion of the root canal space in the middle third of the root of the maxillary left central incisor. Considering the pulp tests indicate vitality and the radiographic evidence of resorption, the most probable diagnosis is internal resorption. The most appropriate treatment for internal resorption when the pulp is vital and the tooth structure allows for it, is to institute root canal therapy. This procedure entails removing the resorptive tissue within the canal, cleaning, disinfecting, and filling the root canal space to prevent reinfection.