What is the ruling on reciting the Tashahhud with connected speech, without vocalizing the last letter's diacritical mark? And is this considered a melodic error (laḥn)?

The fundamental principle for "اللهم صل على محمد وعلى آل محمد" (O Allah, bestow Your prayers upon Muhammad and upon the family of Muhammad) is that it should be recited with the vowel markings. If one wishes to pause, they should make the last letter quiescent. However, making the last letter quiescent when connecting words is not appropriate, though it does not alter the meaning, because the meanings of the words of the Tashahhud (definition) remain intact. This is supported by what is stated in the commentary of Al-Bujairimi regarding examples of minor errors in pronunciation that do not change the meaning.