mypyc: Use widened parameter types in native signatures

[charan-rathore]

Fixes #22221.

With --allow-redefinition, mypy widens a redefined parameter's variable type to the join of the declaration and every redefinition (e.g. n: int later assigned object() becomes union[int, object]). mypyc creates the argument's local register and any closure environment attribute with that widened type, but Mapper.fdef_to_sig built the native signature from the declared Callable type. The generated C entry then received the narrow declared type (CPyTagged) and stored it straight into the wider environment attribute (PyObject *), which fails to compile:

build/__native.c:551:33: error: incompatible types when assigning to type 'PyObject *' {aka 'struct _object *'} from type 'CPyTagged' {aka 'long unsigned int'}

Now each argument's runtime type comes from the parameter variable's type, falling back to the declared Callable type when the variable has none. Without --allow-redefinition the variable type equals the declared type, so existing signatures are unchanged.

One behavior note: the native wrapper for such a function now accepts any object for a widened parameter (mypy still rejects non-conforming calls statically). That matches the flag's semantics - the variable can hold non-declared values at runtime.

Tested:

• The issue's repro compiles now, and the compiled module behaves like the interpreter (f(3) == 2).
• Added a unit test asserting the native signature uses the widened type; it fails without the fix.
• pytest mypyc/test/test_irbuild.py: 783 passed.

Prepared with AI assistance (Instinct assisted).