test(pydantic-ai): Fix assertion for missing tool description

When a tool has no description, the tool_description field should be
present in the span data but have a None value. Update the assertion
to check for field presence rather than field absence.

Also use `is None` instead of `== None` for proper Python style.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Author: ericapisani

tests/integrations/pydantic_ai/test_pydantic_ai.py

@@ -2872,7 +2872,8 @@ def no_docs_tool(a: int, b: int) -> int:
 
     tool_span = tool_spans[0]
     assert tool_span["data"]["gen_ai.tool.name"] == "no_docs_tool"
-    assert SPANDATA.GEN_AI_TOOL_DESCRIPTION not in tool_span["data"]
+    assert SPANDATA.GEN_AI_TOOL_DESCRIPTION in tool_span["data"]
+    assert tool_span["data"][SPANDATA.GEN_AI_TOOL_DESCRIPTION] is None
 
 
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