Reject interpreter pool activity executors

[tconley1428]

Passing an InterpreterPoolExecutor as activity_executor currently takes the shared-thread execution path because it subclasses ThreadPoolExecutor, then fails when activity context objects cross interpreter boundaries.

Reject interpreter pools, including subclasses, during worker configuration with a clear ValueError recommending thread or process pools. Guard detection for Python versions before 3.14, document the unsupported executor, and add a changelog entry.

Validation:

• Both regression cases pass on Python 3.14; the tests skip on older versions.
• poe lint passes, including all three type checkers.
• git diff --check passes.

Refs #1154. This provides explicit rejection; it does not implement subinterpreter support.