refactor: fix FutureWarning by replacing datetime.utcnow() with timezone-aware alternative

Replaces deprecated datetime.datetime.utcnow() with datetime.datetime.now(timezone.utc)
in accordance with Python 3.12+ deprecation warnings.

src/cleveragents/domain/models/observability/llm_trace.py

@@ -133,7 +133,7 @@ class LLMTrace(BaseModel):
         description="Error message if the call failed",
     )
     timestamp: datetime = Field(
-        default_factory=datetime.utcnow,
+        default_factory=lambda: datetime.now(datetime.UTC),
         description="When the trace was recorded",
     )
 

src/cleveragents/domain/models/observability/metrics.py

@@ -218,7 +218,7 @@ class MetricEntry(BaseModel):
         description="ULID of the plan this metric belongs to",
     )
     timestamp: datetime = Field(
-        default_factory=datetime.utcnow,
+        default_factory=lambda: datetime.now(datetime.UTC),
         description="When the observation was recorded",
     )
     labels: dict[str, Any] = Field(