Fix builtin browser failing to start with 'NoneType' object has no attribute 'browser_type'

[Liang-HZ]

Summary

crwl browser start and BrowserProfiler.launch_builtin_browser() fail unconditionally with:

Error starting builtin browser: 'NoneType' object has no attribute 'browser_type'

ManagedBrowser.__init__ still declares — and documents — the legacy keyword
arguments (browser_type, user_data_dir, headless, host, debugging_port,
cdp_url), but its body reads every value from 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   # AttributeError when None

launch_builtin_browser() is the one caller that still passes only the legacy
kwargs, so it raises every time. The two other call sites in the same file were
already migrated to browser_config= (the old kwargs are still sitting there
commented out), which suggests this one was simply missed during the refactor.

Fix

Two small changes:

1. browser_profiler.py — launch_builtin_browser() now builds a BrowserConfig
   and passes it, matching the two sibling call sites in the same file.
2. browser_manager.py — ManagedBrowser.__init__ falls back to the documented
   legacy keyword arguments when browser_config is None, instead of raising an
   opaque AttributeError. This keeps the published signature honest and protects
   any third-party code still calling it the old way.

Reproduction

Before:

$ crwl browser start
Error starting builtin browser: 'NoneType' object has no attribute 'browser_type'

After:

$ crwl browser start
[BUILTIN]. ℹ Builtin browser launched at CDP URL: http://localhost:9222
╭─────────────────────── Builtin Browser Start ───────────────────────╮
│ Builtin browser started successfully                                │
│ CDP URL: http://localhost:9222                                      │
╰─────────────────────────────────────────────────────────────────────╯

crwl browser status / crwl browser stop and BrowserConfig(browser_mode="builtin")
work against the started instance afterwards.

Also verified that constructing ManagedBrowser(...) with only the legacy kwargs
no longer raises, and that passing browser_config= behaves exactly as before.

Related issues

• [Bug]: error while starting browser service #1139 — "error while starting browser service", same error message, reported
  against 0.6.3 via crwl browser start. Closed, but the failure still reproduces
  on main.
• [Bug]: TypeError: 'NoneType' object has no attribute 'browser_type' when creating a new browser profile #1142 — closed as a duplicate of [Bug]: error while starting browser service #1139.

Verified against main at the time of writing: both the missing browser_config
in launch_builtin_browser() and the unconditional browser_config.* reads in
ManagedBrowser.__init__ are still present.