Fix constructor binding when function has optional arguments

[krystophny]

Fixes #373.

convert_derived_type_arguments locates the auto-generated ret_ return argument of a fortranconstructor by taking the first argument with the optional attribute. FunctionToSubroutineConverter inserts ret_ before the first optional argument, so when the constructor function has user-defined optional arguments the first optional is a user argument and the assertion sub.arguments[j].name[:4]=='ret_' fires. Find the ret_ argument by name prefix instead.

Adds examples/issue373_optional_constructor covering the interface widget_t constructor idiom with an optional argument, both present and absent.

Verification

Test fails on main

$ cd examples/issue373_optional_constructor && make test
  File ".../f90wrap/transform.py", line 562, in convert_derived_type_arguments
    assert(sub.arguments[j].name[:4]=='ret_')
AssertionError
make: *** [Makefile:24: f90wrap_main.f90] Error 1

Test passes after fix

$ cd examples/issue373_optional_constructor && make test
python tests.py
..
----------------------------------------------------------------------
Ran 2 tests in 0.000s

OK

Unit tests (pytest test/): 38 passed; the 2 pre-existing test_parse_dnad failures (missing test/samples/DNAD.fpp) are addressed separately. Related examples (keep_single_interface, interface, derivedtypes, fortran_oo, extends) all pass.

@jameskermode ready for review.

[krystophny]

Wheel CI failed for cp312/cp313 on Linux: numpy 2.5 ships only manylinux_2_28 wheels, so the default manylinux2014 container built numpy from source and failed its gcc >= 10.3 check. Included a one-line workflow fix building Linux wheels in the manylinux_2_28 image.

[krystophny]

The build (3.x) jobs failed in the direct-c pass: the new example calls the constructor without its optional argument, and on master the direct-c backend passes 0 instead of an absent pointer (#369), so present(scale) is true and the default branch never runs. Included the identical direct-c fix from #370 so this PR is green independently — either merge order is clean.