Avoid wrapping methods that return Signal

[JosephDemarest]

Summary

• detect declared Signal return metadata before generating a doubled wrapper
• report an actionable GUT error and leave the method inherited instead of generating an await that can stall
• document ignore_method_when_doubling and the limitation for untyped Signal-returning methods

Verification

• focused regression on Godot 4.6.3 / GUT 9.6.1: 2 tests passed, 6 assertions
• complete headless suite: 248 scripts, 1,745 tests, 1,742 passing, 7 existing pending/risky, 3,036 assertions; exit code 0
• GDScript LSP: no errors in the changed source or regression test
• gdlint on the new regression test: no problems
• git diff --check: clean

Fixes #833