Fix UnboundLocalError in streaming API URL parsing

[elnikkis]

This PR fixes an UnboundLocalError that occurs when calling stream_public() and other streaming endpoints.

When calling mastodon.stream_public(), the following error is raised:

  Traceback (most recent call last):
    File "/home/nicky/repos/mastodon-crawler/mastodon_crawler/get_stream.py", line 151, in <module>
      crawl(args)
      ~~~~~^^^^^^
    File "/home/nicky/repos/mastodon-crawler/mastodon_crawler/get_stream.py", line 82, in crawl
      handle = func(listener, run_async=True, reconnect_async=True)
    File "/home/nicky/repos/mastodon-crawler/.venv/lib/python3.13/site-packages/decorator.py", line 235, in fun
      return caller(func, *(extras + args), **kw)
    File "/home/nicky/repos/mastodon-crawler/.venv/lib/python3.13/site-packages/mastodon/versions.py", line 50, in wrapper
      return function(self, *args, **kwargs)
    File "/home/nicky/repos/mastodon-crawler/.venv/lib/python3.13/site-packages/mastodon/streaming_endpoints.py", line 37, in stream_public
      return self.__stream(base, listener, run_async=run_async, timeout=timeout, reconnect_async=reconnect_async, reconnect_async_wait_sec=reconnect_async_wait_sec)
             ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    File "/home/nicky/repos/mastodon-crawler/.venv/lib/python3.13/site-packages/mastodon/internals.py", line 412, in __stream
      url = self.__get_streaming_base()
    File "/home/nicky/repos/mastodon-crawler/.venv/lib/python3.13/site-packages/mastodon/internals.py", line 389, in __get_streaming_base
      parse = urlparse(instance["urls"]["streaming_api"])
                       ^^^^^^^^
  UnboundLocalError: cannot access local variable 'instance' where it is not associated with a value

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.79%. Comparing base (e36237f) to head (2ac51ca).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #433   +/-   ##
=======================================
  Coverage   88.79%   88.79%           
=======================================
  Files          35       35           
  Lines        4821     4821           
=======================================
  Hits         4281     4281           
  Misses        540      540           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[halcy]

huh! weird that that slipped through testing. I‘ll have a look next time I am working on mastopy (possibly tomorrow, though no promises) and merge and add a test to catch this if possible.

Thank you a lot for finding and fixing this, in any case!