How can I add in rain into a photo

Instead of using generative AI, I would probably make what appears to be rain. Here's how:

1. Make a new layer.
2. Make sure the foreground and background colors are set to the default black and white (click the small black and white icon above the colors in the toolbar).
3. Go to Filter > Render > Fibers... and set bump up the Variance and Strength as needed to get some good contrast.
4. Rotate the vertical fibers 90° so that they are horizontal.
5. Go to Filter > Stylize > Wind... and set the Method to Blast.
6. Rotate the layer -90° so that it's vertical again.
7. Invert the layer so that it's mostly black (Image > Adjustments > Invert).
8. Set the blend mode for the layer to Lighten or Screen.
9. Reduce the opacity of the layer.
10. Skew and scale if you'd like the rain to appear to slant.