fix: close browser contexts from snapshot

[nightcityblade]

Summary

Fixes #1999

Close browser contexts from a stable snapshot so shutdown is not interrupted if contexts_by_config changes while a context is being closed.

List of files changed and why

• crawl4ai/browser_manager.py - Iterate over a snapshot of context values in BrowserManager close paths.
• tests/browser/test_browser_manager_close.py - Add a regression test for context-map mutation during close.

How Has This Been Tested?

• python -m pytest -q tests/browser/test_browser_manager_close.py

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
• 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