The execution of SELECT statements with ANN ordering (vector search) was
previously implemented within `indexed_table_select_statement`. This was
not ideal, as vector search logic is independent of secondary index selects.
This resulted in unnecessary complexity because vector search queries don't
use features like aggregates or paging. More importantly,
`indexed_table_select_statement` assumed a non-null `view_schema` pointer,
which doesn't hold for vector indexes (where `view_ptr` is null).
This caused null pointer dereferences during ANN ordered selects, leading
to crashes (VECTOR-179). Other parts of the class still dereference
`view_schema` without null checks.
Moving the vector search select logic out of
`indexed_table_select_statement` simplifies the code and prevents these
null pointer dereferences.
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