[mypyc] Fix @property getter memory leak

[VaggelisD]

Issue

Mypyc can incorrectly borrow references from property getters (owned references).

The borrow mechanism is safe for struct field access since they return a pointer into the object's memory; Property getters on the other hand are method calls that return new owned references i.e the caller must DECREF them. If mypyc mistakes a property for a struct field, it skips the DECREF, leaking one reference per call.

I think this affects any expression context that enables borrowing (comparisons, isinstance, is None checks) when the borrowed expression is a property access on a cross-module class. We discovered this through OOM issues in SQLGlot, where isinstance(self.this, Func) (this is a @property) leaked on every call.

Repro

  # base.py:
  class Bar:
      pass  
          
  class Foo:
      def __init__(self) -> None:
          self.obj: object = Bar()

      @property
      def val(self) -> object:
          return self.obj  
                                                                  
  # derived.py:
  from base import Foo, Bar
                           
  def check(foo: Foo) -> bool:
      return isinstance(foo.val, Bar)

  # test_leak.py: 
  import sys
  from base import Foo 
  from derived import check    
                           
  foo = Foo()
  init = sys.getrefcount(foo.obj)
  for _ in range(100):     
      check(foo) 
  print(f"Leaked refs: {sys.getrefcount(foo.obj) - init}")

Compile both with mypyc passing in PYTHONHASHSEED=3 prints Leaked refs: 100.

Root cause

1. is_native_attr_ref() decides if an attribute access can safely borrow by checking has_attr(name) and not get_method(name) i.e "if there's an attribute but no method, it's a struct field".
2. Although has_attr() is always fully populated, get_method() depends on whether the class's module (base in this case) has been compiled yet.
3. Modules within an SCC are compiled in nondeterministic order; If the module defining the property hasn't been compiled yet -> get_method() returns None -> is_native_attr_ref incorrectly treats the property as a borrowed struct field.

I think there's a broader issue here: Even when derived.py imports from base.py (no cycle), mypyc can place them in the same SCC and compile derived before base. Since ir.methods is only populated when a module's function bodies are compiled, any code that reads ir.methods (i.e get_methods) during compilation of a different module in the same SCC could have similar order-dependent bugs.

The suggested fix

Check ir.attributes directly (struct fields only, always populated) instead of the unreliable get_method. I believe this is the getter-side counterpart to #21095 which fixed the same class of bug for property setters.