browser: pass _default_context_options into new_context

[SAY-5]

Fixes #414.

Problem

LocalChromiumBrowser accepts a context_options kwarg in its constructor, merges it into self._default_context_options, and stores it correctly. But the base class Browser._setup_session_from_browser ignores the dict and calls new_context() with no arguments:

if isinstance(browser_or_context, PlaywrightBrowser):
    session_browser = browser_or_context
    session_context = await session_browser.new_context()   # ← drops options
    session_page = await session_context.new_page()

Result: viewport, user_agent, storage_state, locale, timezone_id, geolocation, permissions, color_scheme, …all silently ignored. The reporter's proof script shows viewport = 1280x720 (Playwright default) instead of the requested 1920x1080.

Particularly impactful for storage_state, the standard Playwright mechanism for reusing authentication across browser contexts. There is no public-API workaround; users today have to subclass and override _setup_session_from_browser.

Fix

• Initialize _default_context_options: Dict[str, Any] = {} on the base Browser.__init__ (empty default → no behaviour change for subclasses that don't populate it).
• Pass **self._default_context_options into session_browser.new_context(...) in the non-persistent branch of _setup_session_from_browser.

LocalChromiumBrowser already populates the dict during start_platform(), so users who passed context_options to its constructor now actually see those options applied.

Back-compat

• The persistent-context branch is untouched (it receives an already-constructed context from the caller).
• Subclasses that don't set _default_context_options get the empty-dict default, so new_context(**{}) is a no-op equivalent to today's new_context().

Signed off per DCO.