Add missing context proptery to MockSpan (#6549)

The property context should return a mocked context and not a regular one

stubs/opentracing/opentracing/mocktracer/span.pyi

@@ -25,6 +25,8 @@ class MockSpan(Span):
     ) -> None: ...
     @property
     def tracer(self) -> MockTracer: ...
+    @property
+    def context(self) -> SpanContext: ...
     def set_operation_name(self: Self, operation_name: str) -> Self: ...
     def set_tag(self: Self, key: str, value: str | bool | int | float) -> Self: ...
     def log_kv(self: Self, key_values: dict[str, Any], timestamp: float | None = ...) -> Self: ...