fix: CachedSpanFetcher no longer caches empty results permanently

[Curtis Galione (cjgalione)]

Summary

• CachedSpanFetcher.get_spans() was setting _all_fetched = True even when the fetch returned no spans, permanently caching the empty result and preventing future re-fetches
• Now only marks as fully fetched when the cache is non-empty, so subsequent calls will retry and pick up spans once they become available

Test plan

• Updated test_handle_empty_results to verify re-fetch occurs when first call returned empty
• Added test_empty_then_populated_refetches to confirm spans are returned on retry
• Added test_empty_results_with_type_filter to confirm type-filtered path also handles empty results
• Verified existing caching tests still pass (non-empty fetches are still cached)