Fix RecursionError when serializing Enum/IntFlag parameter values

[marcosfrenkel]

Summary

Station snapshots (and parameter gets) containing IntFlag/Enum values crashed
the server with RecursionError: maximum recursion depth exceeded on the send
path. This happens with real drivers such as the Yokogawa GS200, whose
status/event-register parameters return IntFlag instances.

Root cause

Starting with Python 3.11, Flag/IntFlag members are iterable, and a
single-bit member iterates to a one-element sequence containing itself:

>>> list(StatusByte.EAV)
[<StatusByte.EAV: 4>]
>>> list(StatusByte.EAV)[0] is StatusByte.EAV
True

The serializers (dict_to_serialized_dict / iterable_to_serialized_dict)
recurse into anything that is a non-str Iterable. A flag value matched that
check and recursed into itself forever, raising RecursionError from encode()
on the server's send path — so the client never got a response.

Fix

blueprints.py:

• Treat any Enum as a scalar in dict_to_serialized_dict and
  iterable_to_serialized_dict (serialize its .value), placed before the
  generic Iterable check so flag members never recurse.
• For a top-level Enum/IntFlag message (the return of a parameter get),
  serialize it with its _class_type via the new _convert_enum_to_dict helper
  so the client reconstructs the actual enum instance — not just a bare int.

Tests

New test/pytest/test_enum_serialization.py:

• Unit coverage that single-bit, multi-bit, and plain enums serialize to their
  value without recursion.
• encode(ServerResponse(...)) over a snapshot reproduces the original failing
  path and asserts valid JSON.
• Round-trip tests that a top-level enum message reconstructs back to the real
  enum type.
• End-to-end client/server tests (get, instrument snapshot, full station
  snapshot) using a new DummyInstrumentWithFlags dummy instrument that mirrors
  the GS200 status registers.

Notes

A nested flag inside a snapshot is serialized lossily to its integer value
(snapshots are display-oriented), whereas a parameter get round-trips back to
the actual enum type via _class_type.