Add `plugins` option support for TRACE_START and TRACE_LIST

…irdSQL#69.

The v5 iAttachment.detach() override dropped the `if self._refcnt:` guard
that iAttachment_v4.detach() has. During cyclic garbage collection the
iAttachment's own iReferenceCounted.__del__ can run release() first (freeing
the native interface and setting _refcnt to 0); Connection.__del__ then calls
detach() on the already-released interface, dereferencing freed memory and
crashing with an access violation. Restore the guard so detach() is a no-op
once the interface has been released.

Adds a regression test that abandons connections without close() and forces
cyclic GC, asserting no access violation escapes the finalizer.

FirebirdSQL#68 added a `plugins` keyword-only parameter to
ServerTraceServices.start() but without a default value, making it
required. This breaks every existing caller -- including this project's
own tests/test_server.py::test_trace -- with:

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

The method body already treats the argument as optional
(`if plugins is not None:`) and its docstring documents it as FB6+ only,
so restore the `= None` default. Adds a regression test asserting the
parameter keeps a default of None.

…ns-default

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

Fix segfault in garbage collection of abandoned connection (FirebirdSQL#69)