fix(storage): propagate vector lookup errors

[hsusul]

Summary

• return None from vector bucket and index lookups only for HTTP 404 responses
• propagate authorization, rate-limit, and server errors as StorageApiError
• cover the behavior for both generated sync and async clients

Problem

get_bucket() and get_index() caught every StorageApiError, so a service failure such as HTTP 500 was indistinguishable from a missing resource.

Solution

Keep the existing optional-result behavior for HTTP 404, and re-raise every other API error. The sync implementation and tests are generated from the async source.

Tests

• regression first: the new bucket and index tests each failed on clean main because no StorageApiError was raised
• uv run --package storage3 pytest -o addopts= src/storage/tests/_async/test_vectors.py src/storage/tests/_sync/test_vectors.py -q — 6 passed
• make tests in src/storage — 122 passed; mypy passed
• uv run ruff check src/storage — passed
• uv run ruff format --check src/storage — 36 files already formatted
• uv build --package storage3 — source distribution and wheel built successfully
• git diff --check — passed

Fixes #1649

cc @olirice @silentworks