Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@crates/capi/src/abstract_.rs`:
- Around line 191-199: PyObject_Format currently dereferences format_spec
unconditionally, which crashes when callers pass NULL. Update the
PyObject_Format function to detect a null format_spec and treat it as an empty
PyStr before calling try_downcast_ref::<PyStr>(vm), while leaving the obj
handling and vm.format call unchanged.
- Around line 139-147: PyVectorcall_Call is currently routed through
PyObject_Call, which bypasses the vectorcall fast path and can recurse when used
as the callable’s call bridge. Update PyVectorcall_Call in abstract_.rs to
dispatch via the vectorcall entrypoint instead of the normal call slot, using
the existing vectorcall-related helpers/types in this module so the tp_call
bridge preserves vectorcall behavior.

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@crates/capi/src/abstract_.rs`:
- Around line 225-228: In PyObject_Format within abstract_.rs, the current
try_downcast_ref::<PyStr> path returns a generic downcast error for non-string
format_spec values, but this C API should map that case to SystemError to match
CPython. Update the handling around format_spec.as_ref() so non-str specs are
explicitly translated into a SystemError before continuing, while keeping the
existing empty_str fallback for None.