[BUG] ImageBuf::nmiplevels() returns 0

OpenImageIO 3.1.14.0

Only cached version of ImageBuf object returns correct number of mipmaps.
```
buffer = ImageBuf(imageFileName, subimage, mip, cache);
nmips = buffer.nmiplevels(); // Works as expected
```

```
buffer = ImageBuf(imageFileName, subimage, mip);
nmips = buffer.nmiplevels(); // Always returns zero
```

Same for both exr and tiff backends.

Could be the same issue: https://github.com/AcademySoftwareFoundation/OpenImageIO/discussions/4984

Start by reproducing the issue with the cached and uncached ImageBuf constructors, using both EXR and TIFF inputs, and compare their nmiplevels() results. Trace the ImageBuf::nmiplevels() entry point and the constructor paths, then verify that both forms report the correct mipmap count.

Written by the indexing model from the issue text.