docs: add memory query tracing example

[RobinALG87]

Summary

Small focused improvement for tracing memory query operations.

Changes

• Add a minimal Phoenix OTEL example for a memory.query span.
• Capture query, result count, top score, hit/miss, latency, and retrieval document attributes.

Validation

• python -m py_compile examples\memory_query_span.py passed
• python -m ruff check examples\memory_query_span.py passed

Notes

Kept intentionally small to make review easy.

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