The Rust guard had two duplicated response-shape extraction blocks in response_items.rs and duplicated PR merged-state detection in both helpers.rs and backend.rs. This change consolidates both paths and preserves the existing response-shape handling while fixing the merged_at: null → merged: true fallback case.

• Consolidate response item extraction

  • Add extract_items_slice(response, list_field) in labels/response_items.rs
  • Reuse it for both PR and issue labeling paths
  • Preserve the existing extraction order:
    1. root array
    2. items
    3. type-specific envelope (pull_requests / issues)
    4. GraphQL nodes
    5. GraphQL single object
    6. bare REST singleton object

• Centralize PR merged-state detection

  • Add shared is_pr_merged(item) in labels/helpers.rs
  • Replace inline merged checks in:
    • pr_integrity(...)
    • get_pull_request_facts_with_callback(...)
  • Remove raw "merged_at" / "merged" handling from backend.rs

• Behavioral cleanup

  • Correct merged detection when merged_at is present but null, so the logic now falls back to merged: true as intended

• Targeted coverage

  • Add focused tests for:
    • type-specific item envelopes
    • singleton REST object promotion
    • is_pr_merged timestamp and boolean fallback behavior
    • backend PR fact extraction via merged boolean fallback

fn extract_items_slice<'a>(response: &'a Value, list_field: &str) -> &'a [Value] {
    if let Some(arr) = response.as_array() {
        arr.as_slice()
    } else if let Some(arr) = response.get("items").and_then(|v| v.as_array()) {
        arr.as_slice()
    } else if let Some(arr) = response.get(list_field).and_then(|v| v.as_array()) {
        arr.as_slice()
    } else if let Some(nodes) = extract_graphql_nodes(response) {
        nodes.as_slice()
    } else if let Some(obj) = extract_graphql_single_object(response) {
        std::slice::from_ref(obj)
    } else {
        &[]
    }
}