List operation should skip objects with invalid paths instead of returning 503

## Problem

When a backend bucket contains an object with an invalid key (e.g. `dynamical/asos-parquet//` — double slash / empty path segment), the list operation fails with a 503 ServiceUnavailable error:

```
backend error: Encountered object with invalid path: Path "dynamical/asos-parquet//" contained empty path segment
```

This makes the entire listing unusable even though only one object has a bad key.

## Expected behavior

The proxy should skip objects with invalid paths and return the remaining valid results. A warning log is appropriate, but the response should succeed.

## How to reproduce

List a bucket that contains an object key with an empty path segment (double slash):

```
aws s3 ls s3://dynamical/asos-parquet/ --endpoint https://data.source.coop
```

## Context

These invalid keys can be created by tools that accidentally add trailing or double slashes. The proxy shouldn't let one bad object break listing for an entire prefix.