refactor(profiling): annotate native-heap live-heap arming local

Annotate the `mode` local introduced in _ProfilerInstance._start_service
when reporting which native-heap mode (live-heap vs allocation-only) was
armed. The live_heap_enabled activator additions and smoke-test assertions
in this PR were already fully annotated.

Author: vlad-scherbich

ddtrace/profiling/profiler.py

@@ -396,7 +396,7 @@ def _start_service(self) -> None:
                     # live-heap (ddheap:free + retain flagging) is a build-time
                     # property of the cdylib, not a runtime toggle; report which
                     # mode was actually armed for observability.
-                    mode = "live-heap" if heap_gotter.live_heap_enabled() else "allocation-only"
+                    mode: str = "live-heap" if heap_gotter.live_heap_enabled() else "allocation-only"
                     LOG.debug("Native heap profiling armed (GOT overrides installed, %s)", mode)
                 else:
                     LOG.debug("Native heap profiling requested but GOT overrides were not installed")