fix error on definition for implicit abstract classes

[asukaminato0721]

fix #1200

[yangdanny97]

Back to you with some comments

[asukaminato0721]

Seems that 6f6634d will fail...

extends_abc() returns true even when the class inherits an ABC only indirectly, because the flag is propagated through base metadata.

Child(Base) now satisfies extends_abc() == true, so the solver skips emitting ErrorKind::ImplicitAbstractClass.

failures:

---- test::abstract_methods::test_abstract_async_iterator stdout ----
DEBUG Running with 3 threads (5 MiB stack size)
DEBUG Running epoch 1 of run 0
DEBUG Committing transaction
ERROR Abstract methods for async generators should use `def`, not `async def` [bad-function-definition]
   --> pyrefly/lib/test/abstract_methods.rs:240:15
    |
240 |     async def foo(self) -> AsyncIterator[int]:  # E: Abstract methods for async generators should use `def`, not `async def`
    |               ^^^
    |
Error: Expectations failed for in-memory pyrefly/lib/test/abstract_methods.rs: expected 2 errors, but got 1

---- test::abstract_methods::test_abstract_property stdout ----
DEBUG Running with 3 threads (5 MiB stack size)
DEBUG Running epoch 1 of run 0
DEBUG Committing transaction
ERROR Cannot instantiate `Child` because the following members are abstract: `processor` [bad-instantiation]
   --> pyrefly/lib/test/abstract_methods.rs:225:10
    |
225 | x = Child()  # E: Cannot instantiate `Child`
    |          ^^
    |
Error: Expectations failed for in-memory pyrefly/lib/test/abstract_methods.rs: expected 2 errors, but got 1

---- test::abstract_methods::test_inherited_abstract_method stdout ----
DEBUG Running with 3 threads (5 MiB stack size)
DEBUG Running epoch 1 of run 0
DEBUG Committing transaction
ERROR Cannot instantiate `Child` because the following members are abstract: `method` [bad-instantiation]
   --> pyrefly/lib/test/abstract_methods.rs:102:14
    |
102 | child = Child()  # E: Cannot instantiate `Child`
    |              ^^
    |
Error: Expectations failed for in-memory pyrefly/lib/test/abstract_methods.rs: expected 2 errors, but got 1

---- test::abstract_methods::test_partial_implementation stdout ----
DEBUG Running with 3 threads (5 MiB stack size)
DEBUG Running epoch 1 of run 0
DEBUG Committing transaction
ERROR Cannot instantiate `Partial` because the following members are abstract: `method2` [bad-instantiation]
   --> pyrefly/lib/test/abstract_methods.rs:144:12
    |
144 | p = Partial()  # E: Cannot instantiate `Partial`
    |            ^^
    |
Error: Expectations failed for in-memory pyrefly/lib/test/abstract_methods.rs: expected 2 errors, but got 1


failures:
    test::abstract_methods::test_abstract_async_iterator
    test::abstract_methods::test_abstract_property
    test::abstract_methods::test_inherited_abstract_method
    test::abstract_methods::test_partial_implementation