Add top-level descriptions response field

[shuchenliu]

solve #12

[Copilot]

Pull request overview

Adds a top-level descriptions field to normalized node responses (when description=true) that aggregates all non-empty descriptions across equivalent identifiers, matching NodeNorm Redis behavior (issue #12).

Changes:

• Replace the single-description extraction in create_normalized_node with a flattened, deduplicated list of all non-empty descriptions and expose it as descriptions while keeping the first one as id.description.
• Change the default of include_descriptions from True to False in create_normalized_node (the only caller passes it explicitly).
• Add pytest cases covering both the descriptions-enabled aggregation and the default-off behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/nodenorm/handlers/normalized_nodes.py	Aggregates and deduplicates descriptions, adds top-level descriptions key, flips default of include_descriptions to False.
tests/test_normalized_nodes_lookup.py	Adds tests verifying description aggregation and default suppression.

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