feat: Finalize B-Tree MVP edge cases

[MichailFalaras]

Problems

• Indexed columns currently do not support NULL values safely.
• B-Tree search keys need basic validation.
• Non-unique secondary indexes can contain duplicate keys, but deletion must identify the specific entry to remove.
• B-Tree root changes must be synchronized with Index.root_page_num by the higher-level index layer.
• Deletion/rebalancing paths still need dedicated tests.

Tasks

• Define and enforce the MVP policy for NULL values in indexed columns.
• Validate num_target_keys > 0.
• Validate num_target_keys <= index->index_key->num_columns.
• [HANDLED BY @ApostolisFalaras] Define how a specific duplicate secondary-index entry is identified during deletion.
• [HANDLED BY @ApostolisFalaras] Update secondary-index deletion to remove only the intended duplicate entry.
• Synchronize Index.root_page_num after B-Tree root split/collapse in the higher-level index layer.
• Add deletion, redistribution, merge, root-collapse, and underflow-propagation tests.
• Add invariant/stress coverage where practical.