fix: await dispatcher cleanup in arun_many streams

[nightcityblade]

Summary

Fixes #2083.

Make the public AsyncWebCrawler.arun_many(..., stream=True) wrapper explicitly own and close the dispatcher stream. Closing the public stream now completes dispatcher cleanup before proxy-session release and before aclose() returns.

List of files changed and why

• crawl4ai/async_webcrawler.py - close the inner dispatcher async generator with contextlib.aclosing().
• tests/async/test_arun_many_stream_cleanup.py - verify that closing the public wrapper synchronously closes its dispatcher stream.

How Has This Been Tested?

• .venv/bin/python -m pytest tests/async/test_arun_many_stream_cleanup.py tests/async/test_dispatchers.py::TestDispatchStrategies::test_memory_adaptive_stream_closure_cleans_up_tasks -q - 2 passed.
• .venv/bin/ruff check --isolated --select E9,F63,F7,F82 --ignore F821 crawl4ai/async_webcrawler.py tests/async/test_arun_many_stream_cleanup.py - passed.
• .venv/bin/black --check --target-version py312 --fast tests/async/test_arun_many_stream_cleanup.py - passed.
• .venv/bin/python -m compileall -q crawl4ai/async_webcrawler.py tests/async/test_arun_many_stream_cleanup.py - passed.
• git diff --check upstream/develop...HEAD - passed.

The full legacy tests/async/test_dispatchers.py module produced 10 passes and one unrelated pre-existing failure: test_monitor_integration still passes removed CrawlerMonitor(max_visible_rows=..., display_mode=...) arguments.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas - N/A; the ownership boundary is expressed directly with aclosing().
• I have made corresponding changes to the documentation - N/A; this restores the documented async-generator cleanup contract.
• I have added/updated unit tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes - the relevant new and existing cleanup tests pass; one unrelated pre-existing test in the broader module fails as noted above.

[nightcityblade]

CI note: the only failing check is the repository's notify-discord job because DISCORD_WEBHOOK is empty for this fork PR. The job exits before any code validation; the focused local cleanup tests and lint checks reported in the PR body are green.