fix: Browser#create_page ensure block masks original error with NoMethodError

[collin-pereira]

Problem

When Browser#create_page is called with new_context: true and a block, an exception raised by contexts.create(...) (for example Ferrum::DeadBrowserError when the browser dies before the context is created) is masked by a NoMethodError from the ensure block.

The local context is nil at that point because the assignment never completed. The ensure block then calls context.dispose unconditionally and raises NoMethodError: undefined method 'dispose' for nil, which propagates in place of the original error. Callers who rescue Ferrum::Error (the gem's documented contract) won't catch the masked NoMethodError, so legitimate errors like DeadBrowserError slip past their handlers.

Fix

The adjacent cleanup of page already uses safe navigation (page&.close) for the same reason; this just makes context.dispose symmetric.

A regression spec stubs browser.contexts.create to raise Ferrum::DeadBrowserError and asserts the original error propagates through the block form. Without the fix it fails with expected Ferrum::DeadBrowserError ... got NoMethodError: undefined method 'dispose' for nil.