bug: preserve FIELDS_STORED_IN_SESSION across OAuth session cleanup #471

This is an extension of the fix introduced in:

• fix: preserve next redirect across OAuth session cleanup #470

Instead of only preserving next, we also preserve FIELDS_STORED_IN_SESSION.

The other PR hits a known issue we've seen in Production, and this hits an issue that might affect openedx-platform?

The following are notes from Claude on this issue. The proposed code change as not been verified:

Param next is the only thing restored, but do_auth() stores more than that before start(). It copies every name in FIELDS_STORED_IN_SESSION (default [], e.g. edx-platform LMS sets ['auth_entry', 'next']) and then separately handles next via redirect_name. Anything else a deployment lists there is flushed by logout() for the same reason next was.

         # logout() flushes the session, which also holds what social-auth just stored for this request:
         # the sanitized redirect and any FIELDS_STORED_IN_SESSION. Carry only those across.
         stored_names = ['next', *self.setting('FIELDS_STORED_IN_SESSION', [])]
         stored_values = {name: self.strategy.session_get(name) for name in stored_names}
         logout(request)
         for name, value in stored_values.items():
             if value is not None:
                 self.strategy.session_set(name, value)

Notes:

• next may appear twice if a deployment also lists it in FIELDS_STORED_IN_SESSION. That should be harmless, since the same value is restored twice.
• do_auth() writes None for listed fields missing from the request. The is not None guard avoids re-creating those keys, and session_get returns None either way.
• The FIELDS_STORED_IN_SESSION could values are still dropped.
  For the test, adding a second field (e.g. FIELDS_STORED_IN_SESSION = ['auth_entry'], set to 'login' before do_start()) and asserting it survives would cover the list handling.