fix(browser): let ManagedBrowser use the arguments it documents

[L4XB]

Summary

ManagedBrowser.__init__ documents eight arguments and then reads every attribute off
browser_config, which defaults to None:

def __init__(self, browser_type="chromium", user_data_dir=None, headless=False,
             logger=None, host="localhost", debugging_port=9222, cdp_url=None,
             browser_config: Optional[BrowserConfig] = None):
    ...
    self.browser_type = browser_config.browser_type      # None when no config was passed

Two consequences, both reproducible:

Constructing it the documented way raises.

>>> ManagedBrowser(browser_type='chromium', user_data_dir='/tmp/x', headless=True, debugging_port=9222)
AttributeError: 'NoneType' object has no attribute 'browser_type'

That is the exact call BrowserProfiler.launch_builtin_browser makes
(browser_profiler.py:1205), and it sits outside the try: below it, so it propagates.
crwl browser start (cli.py:717) and crwl browser restart (cli.py:913) both go through
it, so neither can launch the builtin browser.

With a config, the arguments beside it are silently ignored.

>>> cfg = BrowserConfig(browser_type='chromium', debugging_port=9222)
>>> b = ManagedBrowser(browser_type='firefox', debugging_port=9333, host='0.0.0.0', browser_config=cfg)
>>> b.browser_type, b.debugging_port, b.host
('chromium', 9222, 'localhost')

browser_profiler.py shows the confusion in the repository itself: the call at line 481 has
its arguments commented out next to browser_config=, while the one at line 1020 still
passes user_data_dir, headless and debugging_port alongside a config — they do nothing
there, and only happen to agree with it.

List of files changed and why

• crawl4ai/browser_manager.py — when browser_config is None, build one from the
  arguments. Nothing else changes: a supplied config is still used as-is, so every existing
  call site (browser_manager.py:777, the three in browser_profiler.py, and the tests in
  tests/browser/) behaves exactly as before. The docstring now says which of the two wins.
• tests/unit/test_managed_browser_arguments.py — new.

How Has This Been Tested?

tests/unit/test_managed_browser_arguments.py, 9 cases:

• the all-defaults construction no longer raises
• the exact argument shape launch_builtin_browser uses constructs, with its values intact
• each of the six settings reaches the instance, one parametrized case per argument
• a supplied config still wins, and is still stored as browser_config — the control that
  pins the existing call sites

8 of the 9 fail on main; the one that passes is that control.

tests/unit/test_managed_browser_arguments.py     9 passed
tests/unit                                       same 28 pre-existing failures as main
                                                 (pdf and sitemap optional deps), 58 passed

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