Fix OCI test bucket panic

[yeya24]

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

We are trying to wrap an error which is nil so it ended up return nil.

errors.Wrapf(err, fmt.Sprintf("unsupported OCI provider: %s", provider))

This caused a panic in Thanos E2E test because oci.NewTestBucket tries to reference bkt but both bkt and error is nil.

panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x0 pc=0x105c6b138]

goroutine 1705 [running]:
testing.tRunner.func1.2({0x1068c3ce0, 0x107a4c160})
	/Users/benye/sdk/go1.23.1/src/testing/testing.go:1632 +0x3c0
testing.tRunner.func1()
	/Users/benye/sdk/go1.23.1/src/testing/testing.go:1635 +0x57c
panic({0x1068c3ce0?, 0x107a4c160?})
	/Users/benye/sdk/go1.23.1/src/runtime/panic.go:785 +0xf0
github.com/thanos-io/objstore/providers/oci.NewTestBucket({0x106c2d258, 0x14000035380})
	/Users/benye/go/pkg/mod/github.com/thanos-io/[email protected]/providers/oci/oci.go:417 +0x2c8
github.com/thanos-io/objstore/objtesting.ForeachStore.func10(0x14000035380)
	/Users/benye/go/pkg/mod/github.com/thanos-io/[email protected]/objtesting/foreach.go:178 +0x48
testing.tRunner(0x14000035380, 0x140036e2da0)
	/Users/benye/sdk/go1.23.1/src/testing/testing.go:1690 +0x1a8
created by testing.(*T).Run in goroutine 24
	/Users/benye/sdk/go1.23.1/src/testing/testing.go:1743 +0x668
=== RUN   TestBucketStore_e2e/filesystem

Verification