This passage sharply divides those who believe that Jesus is God from those who say Jesus was created by God, and respective answers demonstrate this fact. This being a hermeneutical question, it it hoped that the words in the test will provide the answer (as opposed to interpretations about the text). Alas, they do not. No amount of examination of Greek words will make the slightest difference here, though some try to make much of distinctions between 'spirit' and 'soul' when that is a red herring.
The essential point in this passage of text is whether Stephen was praying to the resurrected Christ (in heaven) or to God the Father (also in heaven). This is because prayer is the most fundamental means of worship that there is. If a Christian was locked away in solitary confinement, possibly the only form of worship he or she could carry out would be prayer - unless they were also able to sing God's praise in worship.
Therefore, I am not going to make a meal out of this text; I'm simply going to point out that when Stephen was given a vision of both the Father and Christ, together, in heaven, Stephen deliberately chose to address Christ in prayer - not the Father.
Further, Stephen took Jesus' own dying words from the cross, "Father, into your hands I commit my spirit" (Luke 23:46), and turned them into prayer to Christ, followed by a virtual repetition of Jesus' other words from the cross, "Father forgive them, for they know not what they do" (vs. 34) thus:
"[Stephen] being full of the Holy Spirit, looked up steadfastly into
heaven, and saw the glory of God, and Jesus standing at the right hand
of God, and said, 'Behold, I see the heavens opened, and the Son of
man standing on the right hand of God.'... Then [as the stones started
to be hurled at him] 'Lord Jesus, receive my spirit.' And he kneeled
down, and cried with a loud voice, 'Lord, lay not this sin to their
charge.' And when he had said this, he fell asleep [in death]." (Acts
Of note is the fact that Saul of Tarsus had consented to this murder (Acts 8:1), which acted as a trigger for him viciously persecuting Christians. This Saul, who became Paul, was a witness, so this is not fanciful conjecture on the part of other Christians.
So, let the text speak for itself, and the answer is clear that although Stephen could have addressed his prayer of worship to the Father, he chose to address it to the risen Christ. Prayer - being an act of worship - proves that Stephen was worshiping Christ by that prayerful address.