build(deps): move ddtrace to the 4.x line

`ddtrace.tracer` was removed in the 4.x line, so the TYPE_CHECKING import is
retargeted at `ddtrace.trace.Tracer`. The `docker/build_from_pip` image pinned
ddtrace==2.19.0 independently of the lock; that pin is moved to the version the
lock now resolves.

Verified against the installed 4.12.2: `ddtrace.trace.Tracer`,
`ddtrace.patch_all` (proxy_server startup) and `ddtrace.profiling.Profiler`
(proxy_server profiling) all resolve.

Code companion for the ddtrace floor raise in the accompanying dependency
commit; the import lives under TYPE_CHECKING so this commit is inert at runtime
on either line.

docker/build_from_pip/Dockerfile.build_from_pip

@@ -36,7 +36,7 @@ RUN uv venv --python python && \
       "opentelemetry-api==1.28.0" \
       "opentelemetry-sdk==1.28.0" \
       "opentelemetry-exporter-otlp==1.28.0" \
-      "ddtrace==2.19.0" \
+      "ddtrace==4.12.2" \
       "sentry-sdk==2.21.0" \
       "mangum==0.17.0" \
       "azure-ai-contentsafety==1.0.0" \

litellm/litellm_core_utils/dd_tracing.py

@@ -10,7 +10,7 @@ from typing import TYPE_CHECKING, Any, Optional, Union
 from litellm.secret_managers.main import get_secret_bool
 
 if TYPE_CHECKING:
-    from ddtrace.tracer import Tracer as DD_TRACER
+    from ddtrace.trace import Tracer as DD_TRACER
 else:
     DD_TRACER = Any