tests: add test coverage for ParameterMap failure_reasons and quality…....

[karnam-ui]

Closes #126
Adds unit tests for ParameterMap covering which covers-

failure_reasons is None by default
failure_reasons correctly stores per-voxel diagnostic strings
Passing voxels have empty string as failure reason
quality_mask shape matches values shape
Mismatched shapes raise an error

Tests expose the current gap where failure_reasons is declared but never popuateted,

[karnam-ui]

I used AI assistance to help me understand the codebase structure and the correct ParameterMap constructor arguments, but I wrote and debugged these tests myself. Initially the tests failed because I was passing 1D arrays — I read the DataValidationError output and fixed the shape to 3D based on looking at the existing test_maps.py file which uses np.random.rand(64, 64, 20).