fix(dto): codegen backend for a nested structure

[abdulhaq-e]

Codegen was generating incorrect syntax for specific data container nesting. The example shown in the test raises the error:

    def _make_function(
        self,
        source_value_name: str,
        return_value_name: str,
        fn_name: str = "func",
    ) -> Callable[[Any], Any]:
        """Wrap the current body contents in a function definition and turn it into a callable object"""
        source = f"def {fn_name}({source_value_name}):\n{self._body} return {return_value_name}"
        ctx: dict[str, Any] = {**self._fn_locals}

        # add the function to linecache, to get better stacktraces when an error occurs
        # otherwise, the traceback within the generated code will just point
        # to '<string>'
        file_name = f"dto_transfer_function_{secrets.token_hex(6)}"
        linecache.cache[file_name] = (
            len(source),
            None,  # mtime: not applicable
            [line + "\n" for line in source.splitlines()],
            file_name,
        )
>       code = compile(source, file_name, "exec")
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E         File "dto_transfer_function_2714053d47d7", line 38
E           source_instance_0['wrapper']_inner_0 = source_instance_0['wrapper'].inner
E                                       ^^^^^^^^
E       SyntaxError: invalid syntax

litestar/dto/_codegen_backend.py:233: SyntaxError

A similar production use case that raised this error:

class CursorPaginationMetadata(Struct):
    limit: int = 50
    next_cursor: str | None = None
    previous_cursor: str | None = None

class PaginatedResponseMetadata(ResponseMetadata):
    pagination: CursorPaginationMetadata

class UsersWithPagination(Struct):
    data: list[User]
    pagination: CursorPaginationMetadata

The code and tests have been entirely generated by AI. I don't understand the codegen backend enough to evaluate this properly. There might be a simpler fix. I'm also not sure if DTO classes other than that for msgspec are affected.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.34%. Comparing base (69a1ad9) to head (8b41b7f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4670   +/-   ##
=======================================
  Coverage   67.33%   67.34%           
=======================================
  Files         292      292           
  Lines       14941    14942    +1     
  Branches     1676     1676           
=======================================
+ Hits        10061    10062    +1     
  Misses       4743     4743           
  Partials      137      137           

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