Title: Embedded _weakref semantics are incomplete for PyO3 weakref compatibility

Using current RustPython tip (d201c48) through the PyO3 RustPython backend, the remaining weakref test failures narrow to _weakref semantics rather than generic backend bring-up.

Observed behavior:

• PyWeakrefReference::upgrade() can work when backed by _weakref.ReferenceType
• ReferenceType.__callback__ behavior appears wrong / missing
• proxy / anyref upgrade paths still fail when routed through _weakref
• these no longer go through stdlib weakref import recursion once the backend switches to _weakref

Representative PyO3 failures:

• types::weakref::reference::tests::python_class::test_weakref_reference_behavior
  • fails on reference.getattr("__callback__") expecting None
• types::weakref::anyref::tests::python_class::test_weakref_upgrade
  • fails because the generic weakref path cannot recover the referent from proxy/reference uniformly

Likely gap:

• RustPython's built-in weakref / weakproxy public behavior does not yet fully match CPython / PyO3 expectations for:
  • ReferenceType.__callback__
  • proxy/reference upgrade interoperability through the C-API-compatible surface
  • possibly PyWeak / PyWeakProxy exposure through _weakref

Relevant RustPython files:

• crates/vm/src/builtins/weakref.rs
• crates/vm/src/builtins/weakproxy.rs
• crates/vm/src/stdlib/_weakref.rs
• crates/vm/src/object/core.rs

If helpful, I can follow up with a smaller RustPython-only repro, but this is currently the main remaining blocker for the PyO3 weakref test cluster.