Allow subclass receivers on overloaded constructors

[Divyansh151005]

Summary

Overloaded ordinary methods and classmethods already skip self/cls superclass validation on overload stubs, so concrete subclass receivers can participate in overload selection. Overloaded __new__ and __init__ were still always validated (should_validate = is_constructor || !is_overload), which rejected valid patterns such as:

class Path:
    @overload
    def __new__(cls: type[AbsPath], path: str) -> AbsPath: ...
    ...

This change treats constructor overload stubs like other overload stubs: skip per-overload receiver superclass checks, while still validating non-overload constructors and implementation signatures.

Fixes #4701

Test Plan

$ export CARGO_TARGET_DIR=/Users/divyanshbarodiya/Desktop/pyrefly-cargo-target
$ cargo test -p pyrefly --lib subclass_receiver
test test::constructors::test_new_overload_subclass_receiver ... ok
test test::constructors::test_init_overload_subclass_receiver ... ok
test result: ok. 2 passed; 0 failed

$ cargo test -p pyrefly --lib bad_receiver_annotation
test test::constructors::test_init_bad_receiver_annotation ... ok
test test::constructors::test_new_bad_receiver_annotation ... ok
test result: ok. 2 passed; 0 failed

$ cargo test -p pyrefly --lib test::constructors::
test result: ok. 90 passed; 0 failed

$ python3 test.py --no-test --no-tensor-shapes --no-conformance --no-jsonschema
Finished successfully

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