fix(tracing): satisfy ruff and pyright on obs_ids imports

CI lint has been red on `next` since c67e177 added this file:

- ruff I001: typing import block not sorted per the repo's isort config
- pyright reportPrivateImportUsage: `tracer` is not re-exported from
  `ddtrace`, so it must be imported from `ddtrace.trace`

The `except ImportError` guard still covers ddtrace being absent, and
`ddtrace.trace.tracer` exists in the pinned ddtrace 4.10.1.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: declan-scale

src/agentex/lib/core/tracing/obs_ids.py

@@ -22,7 +22,7 @@
 from __future__ import annotations
 
 import os
-from typing import Dict, Optional, Tuple
+from typing import Dict, Tuple, Optional
 
 __all__ = ("get_obs_mode", "obs_correlation")
 
@@ -55,7 +55,7 @@ def _lgtm_ids() -> Optional[Tuple[str, str]]:
 
 def _ddtrace_ids() -> Optional[Tuple[str, str]]:
     try:
-        from ddtrace import tracer
+        from ddtrace.trace import tracer
     except ImportError:
         return None
     ctx = tracer.current_trace_context()