Make ServerTraceServices.start() plugins argument optional (regression from #68)

[fdcastel]

Problem

ServerTraceServices.start() cannot be called the way it always has been:

server.trace.start(config=trace_config, name='my_trace')

raises:

TypeError: ServerTraceServices.start() missing 1 required keyword-only argument: 'plugins'

This breaks every existing caller, including this repository's own
tests/test_server.py::test_trace.

Root cause

#68 added a plugins keyword-only parameter to start(), but without a
default value, so it became required:

def start(self, *, config: str, name: str | None=None, plugins: str | list[str] | None) -> int:

The method body already treats it as optional (if plugins is not None:) and
the docstring documents it as "only for FIREBIRD 6+", so it was clearly
intended to be optional.

Fix

Restore the = None default:

def start(self, *, config: str, name: str | None=None, plugins: str | list[str] | None=None) -> int:

Testing

Adds test_trace_start_plugins_is_optional, a server-free regression test
asserting plugins keeps a default of None. It fails on master and passes
with this change; the existing test_trace integration test also passes again.