Warn when activity method is registered without an instance - #1918

What was changed

When building the activity worker, if a registered activity is a plain function whose first parameter is self, the worker now emits a UserWarning. That almost always means someone passed MyActivities.my_method instead of MyActivities(...).my_method, and today that only shows up later as a confusing failure when the activity runs.

It's a warning rather than an error so nothing that currently works (e.g. odd but intentional function signatures) breaks.

Also added a test next to the existing callable-class bad-register test, and a changelog entry under Added.

Why

Closes #377

Checklist

• Added test_workflow_activity_method_unbound_register in tests/worker/test_workflow.py; it fails without the change and passes with it.
• Ran the related tests locally (-k "activity_method or callable_class") and all of tests/worker/test_activity.py, all passing, and confirmed no existing test triggers the new warning.
• ruff check --select I, ruff format --check, pyright and mypy are clean on the changed files.