“وَ” (wa)a joining word meaning “and.” It connects this word to what came before.Corpus grammarCONJ — prefixed conjunction wa (and).
“شَاهِدٍ” (shāhidin)means “And (the) witness” in this verse. It is a noun form. a word describing a doer or an action in progress. Its opening “mu-” is part of this word pattern, not a separate added word.Corpus grammarN — genitive indefinite active noun.