NewContext() and AsyncNewContext() passed ff_version=None through to
generate_context_fingerprint(), so a context's user agent kept the version
fpgen drew (e.g. Firefox/146) while the browser underneath was 152. They
now default ff_version to the major version of Playwright's
Browser.version, which Juggler reports from MOZ_APP_VERSION_DISPLAY, so the
UA always names the browser the page is actually talking to. An explicit
ff_version still wins.
The docstrings said each context gets "its own real fingerprint preset";
the default has been an fpgen draw, with a preset only when one is passed.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
