What does this PR do?

before pagination on the vector store list endpoints always returns the first page of the list, not the page right before the cursor.

The listings filter out everything from the before cursor onwards and then slice [:limit], so they keep the head of what's left instead of the items closest to the cursor. With six vector stores and limit=2, paging forward with after gives [vs_1, vs_2], [vs_3, vs_4], [vs_5, vs_6], but before=vs_5 returns [vs_1, vs_2] instead of [vs_3, vs_4]. A client walking backwards jumps straight to the start. The OpenAI docs describe before=obj_foo as the way "to fetch the previous page of the list", and anthropic_list_models in core/routing_tables/models.py already does it that way (start_index = max(0, end_index - page_limit)).

When the before cursor is found, the page is now [-limit:] of the remaining items. has_more is unchanged: it was already len(remaining) > limit, which is correct for the backward direction. An unknown before cursor keeps today's behavior and returns the head of the list.

Affected listings (same pattern in each):

• VectorIORouter.openai_list_vector_stores (the code path behind GET /v1/vector_stores)
• OpenAIVectorStoreMixin.openai_list_vector_stores
• OpenAIVectorStoreMixin.openai_list_files_in_vector_store (behind GET /v1/vector_stores/{id}/files)
• OpenAIVectorStoreMixin.openai_list_files_in_vector_store_file_batch (behind GET /v1/vector_stores/{id}/file_batches/{batch_id}/files)

Not changed here: ResponsesStore.list_response_input_items has the same slicing, but test_responses_store_input_items_before_pagination asserts the current output (before="before-5", limit=2 in ascending order -> [First, Second]), so it seemed better to ask before changing that behavior. I can open a follow-up if you agree it should match.

Test Plan

New unit tests, one per fixed listing. All of them fail on main with the head-of-list page (for example assert ['vs_1', 'vs_2'] == ['vs_3', 'vs_4']) and pass with this change:

• tests/unit/core/routers/test_vector_io.py::test_list_vector_stores_before_cursor_returns_previous_page
• tests/unit/providers/utils/memory/test_openai_vector_store_mixin.py::TestBeforeCursorPagination (vector stores and vector store files)
• tests/unit/providers/vector_io/test_vector_io_stores_batches.py::test_file_batch_pagination_before_cursor (sqlite_vec, faiss, pgvector, qdrant)

$ uv run pytest tests/unit/core/routers/test_vector_io.py tests/unit/providers/utils/memory/test_openai_vector_store_mixin.py tests/unit/providers/vector_io/test_vector_io_stores_batches.py -k before
# main:        7 failed, 89 deselected
# this branch: 7 passed, 89 deselected

$ uv run pytest tests/unit/
4499 passed, 28 skipped, 3 xfailed

Reproduction against the router and the mixin (mocked storage only):

== VectorIORouter.openai_list_vector_stores (order=asc, limit=2)
forward pages via after: [['vs_1', 'vs_2'], ['vs_3', 'vs_4'], ['vs_5', 'vs_6']]
main:        before='vs_5': got ['vs_1', 'vs_2'] has_more=True; expected ['vs_3', 'vs_4']
this branch: before='vs_5': got ['vs_3', 'vs_4'] has_more=True; expected ['vs_3', 'vs_4']
== OpenAIVectorStoreMixin.openai_list_files_in_vector_store (order=asc, limit=2)
forward pages via after: [['file_1', 'file_2'], ['file_3', 'file_4'], ['file_5', 'file_6']]
main:        before='file_5': got ['file_1', 'file_2'] has_more=True; expected ['file_3', 'file_4']
this branch: before='file_5': got ['file_3', 'file_4'] has_more=True; expected ['file_3', 'file_4']

pre-commit run --files <changed files> passes except mypy, which reports the same 4 errors on untouched main in this environment (src/ogx_api/filters.py:73 and three in openai_responses.py). This diff adds no new ones.