Add pending flag for SoonValue

[shuofengzhang]

What changed

• add a pending property on SoonValue as a clear inverse of ready
• cover the new flag behavior with a small unit test

Why

• the docs and examples already reference soon_value.pending; adding the property aligns behavior with the documented API and avoids AttributeError surprises

Testing

• ✅ pytest -q

[svlandeg]

We don't need this flag, as the inverse .ready already exists. There was one docstring that incorrectly referenced .pending and this has been fixed, cf #269. Closing this one.