Fix TypeGuard/TypeIs validation for functions without explicit return annotation

[AryanBagade]

Summary

Functions returning a TypeGuard/TypeIs value were incorrectly being validated as type guard functions even when they had no explicit return annotation.

For example:

  from typing import TypeGuard

  def is_int(x: int | str) -> TypeGuard[int]:
      return isinstance(x, int)

  class X:
      def __init__(self, param: int | str) -> None:
          self.param = param

      def has_int(self):
          return is_int(self.param)

has_int was being reported as an invalid TypeGuard function ("Type guard functions must accept at least one positional argument") because Pyrefly inferred its return type as TypeGuard[int] and then validated it as a type guard function.

The fix ensures TypeGuard/TypeIs validation only runs when the function has an explicit return annotation.

Fixes #1998

Test Plan

• Added test_typeguard_return_without_annotation and test_typeis_return_without_annotation test cases
• Ran python3 test.py - all tests pass
• Verified existing TypeGuard/TypeIs validation tests still pass (test_typeguard_argument_number, test_typeis_argument_number, test_typeis_subtyping)

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅