Remove memory backend

[chris124567]

Close #132

Marking as draft because I need to add owners to the Sia backend in another PR

[Copilot]

Pull request overview

This PR removes the standalone in-memory S3 “memory backend” and shifts tests/dev tooling to exercise the Sia backend using an ephemeral SQLite store plus an in-memory/mocked Sia SDK, aligning with Issue #132’s goal of avoiding duplicated backend logic.

Changes:

• Replaced the old testutil MemoryBackend with a Sia-backed default test backend (testutil.NewBackend) using SQLite + testutil.MemorySDK.
• Moved the in-memory SDK used by Sia tests into internal/testutil and updated Sia tests to use it.
• Updated SQLite object-part retrieval to also hydrate FileName when fetching a specific multipart part.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
sia/sia_test.go	Drops local MemorySDK test double in favor of testutil.NewMemorySDK and simplifies tester setup.
sia/persist/sqlite/objects.go	Ensures FileName is selected/scanned when retrieving a specific multipart part.
sia/objects_test.go	Updates tests to use testutil.NewMemorySDK and exported counters/flags.
sia/multipart_test.go	Updates Sia multipart tests to use testutil.NewMemorySDK.
s3/objects_test.go	Fixes part-size assertion and disables multi-owner checks due to backend changes.
s3/multipart_test.go	Switches to default tester backend; multi-owner checks are commented out with TODOs.
s3/buckets_test.go	Switches to default tester backend; multi-owner checks are commented out with TODOs.
internal/testutil/testutil.go	Makes testutil.NewTester default to a Sia+SQLite backend.
internal/testutil/backend.go	Removes the old MemoryBackend implementation; adds MemorySDK and NewBackend(tb) helper.
cmd/s3d-tox/main.go	Updates tox harness to run against Sia backend with SQLite + in-memory SDK.

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[Copilot]

These access-control assertions are now commented out, which removes coverage for cross-account authorization (e.g., expecting ErrAccessDenied when using a different access key). Prefer keeping the test as an explicit t.Skip (with a tracking issue) or rewriting it to assert the current intended behavior, so this gap doesn’t silently persist.

Suggested change

	// // upload to a bucket that we don't own
	// otherTester := s3Tester.ChangeAccessKey(t, "foo", "bar")
	// _, err = otherTester.PutObject(t.Context(), bucket, object, bytes.NewReader(data), metadata)
	// testutil.AssertS3Error(t, s3errs.ErrAccessDenied, err)
	t.Run("upload to a bucket that we don't own", func(t *testing.T) {
	t.Skip("TODO: restore cross-account authorization coverage once the sia backend supports owners; expected behavior is ErrAccessDenied for a different access key")
	otherTester := s3Tester.ChangeAccessKey(t, "foo", "bar")
	_, err = otherTester.PutObject(t.Context(), bucket, object, bytes.NewReader(data), metadata)
	testutil.AssertS3Error(t, s3errs.ErrAccessDenied, err)
	})

[Copilot]

The multi-owner test setup is commented out here (and similar blocks appear throughout this file), which drops coverage for authorization behavior. Consider replacing the commented blocks with a skipped subtest (e.g., t.Skip with a link to the owner-support issue) to keep the intent visible while avoiding dead commented code.

[Copilot]

This file now has multi-owner coverage commented out (backend setup + cross-account assertions). To avoid losing long-term authorization coverage, prefer using explicit skipped tests (t.Skip with rationale / tracking issue) rather than leaving commented code blocks.