fix: validate individual values in async generator functions

[gaoflow]

Problem

When @deal.post, @deal.ensure, or @deal.pre was applied to an async generator function, the contracts did not work correctly.

Python's iscoroutinefunction() and isgeneratorfunction() both return False for async generators — only isasyncgenfunction() identifies them. Because _ensure_wrapped had no branch for async generators, they fell through to the synchronous wrapper, which called self.func(*args, **kwargs) and received an async generator object as the result. @deal.post then validated that object instead of each yielded value.

import asyncio, deal

@deal.post(lambda x: x > 0)
async def positive_values(n):
    yield n        # should pass
    yield -n       # should fail

async def main():
    async for v in positive_values(3):
        print(v)   # prints 3 and 6, -3 is never caught

asyncio.run(main())
# No PostContractError raised — the async_generator object itself
# evaluated as truthy and passed the contract silently.

Fix

Add an isasyncgenfunction branch in _ensure_wrapped and a new _run_async_iter method that:

1. Checks pre-conditions before iteration starts.
2. Iterates the underlying async generator with async for.
3. Checks post/ensure conditions after each yielded value.

The change is 31 lines, mirrors the existing _run_iter logic, and adds tests for both @deal.post and @deal.ensure on async generators.

This pull request was prepared with the assistance of AI, under my direction and review.