Fix: lstsq compilation with complex<float> inputs

[deniz-unal]

This fix resolves a compilation error gelsd.tcc:146:25: error: invalid conversion from 'const float*' to 'cxxlapack::FLOAT_COMPLEX*' when linalg::lstsq is called with complex arguments. Note that complex<double> and other types were unaffected.

• A test case that demonstrates the issue is added to test/test_linalg.cpp
• The fix adds const to the relevant interface to correctly match the expected input data type
• The test compiles and passes with this fix