fix: await dispatcher stream cleanup

[nightcityblade]

Summary

Fixes #2083

Explicitly closes the inner dispatcher stream when a caller closes the public arun_many stream, ensuring dispatcher tasks and queue state are cleaned up before proxy-session release.

List of files changed and why

• crawl4ai/async_webcrawler.py - Retain and close the dispatcher stream in the wrapper cleanup path
• tests/async/test_dispatchers.py - Exercise early stream closure through AsyncWebCrawler.arun_many and assert task cancellation and queue cleanup

How Has This Been Tested?

• uv run pytest tests/async/test_dispatchers.py::TestDispatchStrategies::test_arun_many_stream_closure_cleans_up_tasks -q (1 passed)
• Full dispatcher file excluding one unrelated stale CrawlerMonitor constructor test (10 passed, 1 deselected)
• Black check for the changed test file, compileall, and git diff --check

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
• I have made corresponding changes to the documentation (N/A: this is an internal cleanup fix)
• 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 dispatcher tests pass; one unrelated stale monitor test remains)