Raise exception when user-specified reader package is not installed

[haoyu-haoyu]

Fixes #7437

Description

When a user explicitly specifies a reader (e.g., reader="ITKReader") but the required package is not installed, LoadImage now raises an OptionalImportError instead of silently falling back to another reader with only a warning.

Before (current behavior):

# User specifies ITKReader but itk is not installed
loader = LoadImage(reader="ITKReader")
# Only a warning is printed, then PILReader is used silently
# This can lead to unexpected results

After (new behavior):

# User specifies ITKReader but itk is not installed
loader = LoadImage(reader="ITKReader")
# OptionalImportError is raised immediately with a clear message:
# "Required package for reader ITKReader is not installed..."

Default behavior (reader=None) remains unchanged — missing optional packages are still handled gracefully with debug logging, and the system falls back to available readers as before.

Changes

1. monai/transforms/io/array.py: Changed warnings.warn() to raise OptionalImportError() in the user-specified reader registration path (lines 212-216).

2. tests/transforms/test_load_image.py: Added TestLoadImageReaderNotInstalled test class with:

   • test_specified_reader_not_installed_raises: Direct test (runs when itk is NOT installed)
   • test_specified_reader_not_installed_raises_mocked: Mock-based test that always runs, simulating a reader whose package is not installed

Testing

• New unit tests added
• Existing tests unaffected (default reader=None path unchanged)

Signed-off-by: haoyu-haoyu haoyu-haoyu@users.noreply.github.com