Improves invariant calculator tests

[jellybean2004]

Description

This PR refactors and expands the sasinvariant test suite for clarity, consistency, and branch coverage.

• Migrated utest_* modules to pytest-style test modules.
• Added focused tests for:
  • uncertainty propagation (closed-form and hard-coded checks),
  • extrapolation behavior (low/high/both),
  • invalid input handling and error branches,
  • mapper forwarding behavior via dedicated wrapper tests.
• Added shared fixtures in conftest.py.

Coverage impact:

• invariant.py coverage increased from 85% to ~99%.
• invariant_mapper.py coverage increased from 0% to 100%.

How Has This Been Tested?

Ran the tests from the sasinvariant directory and checked the coverage report:

>>> pytest test/sasinvariant/ --cov=src/sas/sascalc/invariant --cov-report=html
...
============================= 114 passed in 2.32s =============================

Review Checklist:

Documentation

• There is nothing that needs documenting
• Documentation changes are in this PR
• There is an issue open for the documentation (link?)

Installers

• There is a chance this will affect the installers, if so
  • Windows installer (GH artifact) has been tested (installed and worked)
  • MacOSX installer (GH artifact) has been tested (installed and worked)
  • Wheels installer (GH artifact) has been tested (installed and worked)

Licensing

• The introduced changes comply with SasView license (BSD 3-Clause)

[Copilot]

Pull request overview

This pull request modernizes the invariant test suite and refactors parts of the invariant implementation to improve validation, typing, and uncertainty handling.

Changes:

• Replaced legacy unittest-style invariant tests with pytest-based equivalents and shared fixtures.
• Added new uncertainty-propagation tests (closed-form checks + hardcoded “oracle” snapshots).
• Refactored invariant.py with typing/ABCs, vectorized q* integration/uncertainty, and improved error handling/logging.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 10 comments.

Show a summary per file

File	Description
src/sas/sascalc/invariant/invariant.py	Refactors invariant core (typing/ABC, vectorized integration, logging, stricter validation).
test/sasinvariant/conftest.py	Adds shared pytest fixtures for synthetic and real invariant datasets.
test/sasinvariant/CalculatorTest.py	Pytest port + expanded calculator behavior/validation tests.
test/sasinvariant/LinearTest.py	Pytest port for extrapolator/linearization tests + additional edge cases.
test/sasinvariant/ExtrapolationTest.py	Pytest port for Guinier/power-law extrapolation behavior.
test/sasinvariant/UseCasesTest.py	Pytest port of “usage perspective” scenarios using bundled data files.
test/sasinvariant/UncertaintyTest.py	New closed-form and hardcoded-value uncertainty tests.
test/sasinvariant/InvariantMapperTest.py	New forwarding/adapter tests for invariant_mapper.
test/sasinvariant/utest_use_cases.py	Removed legacy unittest-based use case tests (superseded by pytest suite).
test/sasinvariant/utest_data_handling.py	Removed legacy unittest-based data-handling tests (superseded by pytest suite).

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[krzywon]

Much improved! The switch to pytest is also welcome here. I don't think we have an issue, but switching all tests from unittest should be a goal to remove the dependency.