fix: explicitly close inner dispatcher stream in arun_many

[Battleplus]

What

The outer result_transformer generator wraps the inner dispatcher.run_urls_stream() generator but doesn't retain a reference to it. When the outer generator is closed, the inner generator's cleanup is deferred to asynchronous-generator finalization.

Now we retain a reference to the inner generator and explicitly call aclose() on it in the finally block, ensuring cleanup completes before the outer generator returns.

Why

Fixes #2083 - AsyncWebCrawler.arun_many() stream closure does not await dispatcher cleanup.

Changes

• Retain reference to inner dispatcher stream generator
• Explicitly call aclose() on inner generator in finally block

Testing

• Verified that stream cleanup now completes before outer generator returns
• Verified that dispatcher tasks are properly cancelled and awaited

Fixes #2083

[Battleplus]

Follow-up audit completed. I rebased the PR onto the required develop branch, kept dispatcher cleanup ahead of proxy-session release even if inner.aclose() raises, and added a regression test through the public AsyncWebCrawler.arun_many(..., stream=True) API.

Validation: python -m pytest tests/async/test_dispatchers.py -q -k stream_closure -> 2 passed.

[Battleplus]

Closing after the cross-PR audit found the earlier #2136, which targets develop and already implements the same inner-stream ownership fix with contextlib.aclosing() plus focused cleanup coverage. Keeping a second implementation would only split review attention.