[BUG] Functions cannot resolve parameter to default when using parameter on an `Optional` type

Bug description

Actual behavior

I get the error "invalid call to 'print_cstr': value passed to 'cstr' cannot be converted from 'None' to 'Optional[CStringSlice[cstr_origin]]', it depends on an unresolved parameter 'cstr_origin._mlir_origin'" when trying the failing examples listed below.

Expected behavior

I'd expect the inclusion of a default parameter value (cstr_origin: ImmutOrigin=ImmutExternalOrigin) to make the first example work. There should be enough information present for the compiler to know that None should cast to Optional[CStringSlice[ImmutExternalOrigin]]. The fact that this doesn't work makes using the new non-nullable versions of CStringSlice and UnsafePointer needlessly verbose.

Steps to reproduce

from std.ffi import CStringSlice

# Doesn't work
def main():
    print_cstr(None)

def print_cstr[cstr_origin: ImmutOrigin=ImmutExternalOrigin](cstr: Optional[CStringSlice[cstr_origin]]):
    if cstr:
        print(cstr.value())
    else:
        print("Null")

# Still doesn't work
def main():
    print_cstr()

def print_cstr[cstr_origin: ImmutOrigin=ImmutExternalOrigin](cstr: Optional[CStringSlice[cstr_origin]]=None):
    if cstr:
        print(cstr.value())
    else:
        print("Null")

# Works but needlessly verbose
def main():
    print_cstr[cstr_origin=ImmutExternalOrigin](None)

def print_cstr[cstr_origin: ImmutOrigin=ImmutExternalOrigin](cstr: Optional[CStringSlice[cstr_origin]]):
    if cstr:
        print(cstr.value())
    else:
        print("Null")

System information

System
------------
       Pixi version: 0.53.0
           Platform: linux-64
   Virtual packages: __unix=0=0
                   : __linux=6.18.7=0
                   : __glibc=2.39=0
                   : __cuda=13.0=0
                   : __archspec=1=zen4
          Cache dir: /home/ryan/.cache/rattler/cache
       Auth storage: /home/ryan/.rattler/credentials.json
   Config locations: No config files found

Global
------------
            Bin dir: /home/ryan/.pixi/bin
    Environment dir: /home/ryan/.pixi/envs
       Manifest dir: /home/ryan/.pixi/manifests/pixi-global.toml

Workspace
------------
               Name: platform-mojo
            Version: 0.1.0
      Manifest file: /home/ryan/Documents/programming/platform-mojo/pixi.toml
       Last updated: 13-05-2026 00:18:55

Environments
------------
        Environment: default
           Features: default
           Channels: conda-forge, https://conda.modular.com/max
   Dependency count: 2
       Dependencies: mojo, libxcb
   Target platforms: linux-64
    Prefix location: /home/ryan/Documents/programming/platform-mojo/.pixi/envs/default

Start by running the failing examples in the issue and compare them with the explicit cstr_origin example that works. Trace parameter-default and Optional generic resolution for CStringSlice, then add or update regression coverage for both None calls. Done means the implicit default resolves the parameter without requiring an explicit generic argument.