fix: always create TreeAhConfig in create_tree_ah_index()

[taimo3810]

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Fixes #6222 🦕

---

Summary

PR #5954 introduced a regression where create_tree_ah_index() fails when leaf_node_embedding_count and leaf_nodes_to_search_percent are not specified, because algorithmConfig is set to None instead of {"treeAhConfig": {}}.

According to the official documentation, algorithmConfig is a required field.

Changes

Reverts the conditional logic in create_tree_ah_index() to always create a TreeAhConfig:

# Before (buggy - after PR #5954)
algorithm_config = None
if (
    leaf_node_embedding_count is not None
    or leaf_nodes_to_search_percent is not None
):
    algorithm_config = TreeAhConfig(...)

# After (fixed)
algorithm_config = TreeAhConfig(
    leaf_node_embedding_count=leaf_node_embedding_count,
    leaf_nodes_to_search_percent=leaf_nodes_to_search_percent,
)

Test

Updated test_create_tree_ah_index_empty_algorithm_config to expect {"treeAhConfig": {...}} instead of None.

All 66 tree-AH related tests pass.

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[taimo3810]

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