Make ServerTraceServices.start() plugins argument optional

#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.

Author: fdcastel

src/firebird/driver/core.py

@@ -5496,7 +5496,7 @@ def __action(self, action: ServerAction, label: str, session_id: int) -> str:
             # response should contain the error message
             raise DatabaseError(response)
         return response
-    def start(self, *, config: str, name: str | None=None, plugins: str | list[str] | None) -> int:
+    def start(self, *, config: str, name: str | None=None, plugins: str | list[str] | None=None) -> int:
         """Start new trace session. **(ASYNC service)**
 
         Arguments:

tests/test_server.py

@@ -22,6 +22,7 @@
 #
 # See LICENSE.TXT for details.
 
+import inspect
 from io import BytesIO
 import pytest
 from packaging.specifiers import SpecifierSet
@@ -158,6 +159,19 @@ def test_get_limbo_transaction_ids(server_connection, db_file):
     ids = server_connection.database.get_limbo_transaction_ids(database=str(db_file))
     assert isinstance(ids, type(list()))
 
+def test_trace_start_plugins_is_optional():
+    """Regression: ServerTraceServices.start() must keep `plugins` optional.
+
+    #68 added `plugins` as a keyword-only parameter but without a default,
+    making it required and breaking every existing caller (including
+    `test_trace` below, and any FB3/4/5 user) with
+    "missing 1 required keyword-only argument: 'plugins'". The method body
+    already treats it as optional (`if plugins is not None:`) and the
+    docstring marks it FB6+ only, so it must default to None.
+    """
+    sig = inspect.signature(driver.core.ServerTraceServices.start)
+    assert sig.parameters['plugins'].default is None
+
 def test_trace(server_connection, db_file, fb_vars):
     trace_config = """database = %s
     {