Use inspect.signature for callable argument checks

[cyphercodes]

Summary

• replace inspect.getfullargspec usage in callable argument checks with a shared get_signature helper
• pass annotation_format=Format.FORWARDREF on Python 3.14+ so Pyramid does not evaluate annotations it does not use
• add regression coverage ensuring these paths no longer call getfullargspec

Fixes #3812.

Tests

• python -m pytest tests/test_util.py::Test_takes_one_arg tests/test_util.py::TestUnboundMethods -q
• python -m pytest tests/test_util.py tests/test_config/test_adapters.py -q
• python -m pytest -q
• git diff --check