Fix MethodBinding/OverloadMapper memory leak (#691) (#2719)

* Fix MethodBinding/OverloadMapper memory leak (#691)

MethodBinding and OverloadMapper held PyObject `target` references that
were not disposed during tp_clear, leaving Python-side refcount drops to
wait on the multi-hop .NET finalizer chain. They also shared the same
C# PyObject instance across mp_subscript/Overloads paths, so freeing one
could free the underlying Python object out from under the others.

- ExtensionType: add virtual OnClear() hook called from tp_clear before
  the GCHandle is released, letting subclasses eagerly drop owned
  Python references.
- MethodBinding/OverloadMapper: override OnClear to dispose `target`.
  (`targetType` is intentionally not disposed since Python types are
  long-lived and tracked by other caches.)
- Take an independent INCREF'd PyObject copy at every site that hands a
  shared target into a new MethodBinding or OverloadMapper, so each
  wrapper owns its own reference.

Result: the three _does_not_leak_memory tests drop from ~485 MB delta
to ~10 KB delta on Python 3.14.

* Tighten leak-test threshold to 10% to actually fail the bug

The previous 90% threshold (0.9 MB/iter against a 1 MB allocation)
documented the issue but did not reproduce it: master leaks
~600-765 KB/iter, which the 0.9 MB threshold accepts as passing.

Drop the threshold to 10% (104 KB/iter). On the 2026-05-09 verification
run with Python 3.14 GIL on linux-aarch64:

  Without fix (master):   ~572-765 KB/iter (FAIL)
  With fix (this branch): ~-500 B/iter     (PASS)

Margin is roughly 6x in either direction across .NET 8 and .NET 10, so
the threshold cleanly separates buggy from fixed states without being
sensitive to GC noise.

* Bugfix and improvements

- Handle the `PyType` reference in `OverloadMapper` and `MethodBinding` in the
  same way as the object reference
- Unconditionally store the `PyType` of the object
- Introduce `NewReference` helper function for the object and type
  passing
- Fix the remaining missing reference count bump for the type
  (`MethodObject`)
- As the count is now correct, `Dispose` the type as well

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Co-authored-by: Benedikt Reinartz <filmor@gmail.com>