feat: Update examples to use create_tracker() factory pattern

[jsonbailey]

Summary

• Updated 6 examples to call config.create_tracker() instead of accessing config.tracker directly, matching the new per-invocation tracker factory pattern from AIC-2207
• Moved tracker creation after the enabled check in completion config examples, since create_tracker is None when the config is disabled
• No changes needed for the 3 high-level API examples (chat_judge, chat_observability, direct_judge) as they don't access the tracker directly

Files changed

• examples/openai_example.py
• examples/bedrock_example.py
• examples/gemini_example.py
• examples/langchain_example.py
• examples/langgraph_agent_example.py
• examples/langgraph_multi_agent_example.py

Test plan

• Run poetry run openai-example and verify metrics are tracked
• Run poetry run chat-judge-example and verify end-to-end flow

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