Page.createIsolatedWorld and do_navigate both hardcoded context id 100
for every isolated world emission. After the second navigation Playwright
expected a fresh incrementing id (101, 102) while obscura kept re-using
100. Runtime.evaluate then failed with 'Cannot find context with
specified id: 101' because the id was never registered in
valid_context_ids and the client's bookkeeping had diverged from the
server's.
Added next_isolated_context_id counter on CdpContext (starts at 100) and
a next_isolated_context() helper that claims an id, registers it in
valid_context_ids, and increments. Both call sites now use it.
SGavrl
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