expanded test suite

[nikosarcevic]

in this PR I have:

• Expanded the existing luminosity-function test suite with additional edge-case and validation coverage.
• Added tests for error handling, broadcasting behavior, numerical edge cases, registry utilities, and wrapper functions.
• Improved coverage across LFKit luminosity-function modules to ensure all source files are exercised by the test suite.
• No functional changes; this PR focuses exclusively on strengthening test coverage and regression protection.

Closes #20