refactor(search): document+guard visibility filter precondition; drop redundant dev-dep

- Remove serde_json from [dev-dependencies] (already in [dependencies])
- Add debug_assert! in search_objects visibility filter as defense-in-depth
- Extend search_objects doc-comment with visibility precondition
- Clarify estimated_total_hits.unwrap_or(0) is safe under offset/limit pagination
- Add brief comment on with_crop_length(20) explaining ~20-word context window

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/search/src/lib.rs

@@ -176,6 +176,12 @@ impl SearchClient {
     /// Full-text query returning display-ready hits with highlighted snippets and the
     /// estimated total match count. `visibility`, when set, filters on the indexed
     /// `visibility` attribute. Pagination is offset/limit.
+    ///
+    /// # Preconditions
+    ///
+    /// When `visibility` is `Some`, the value must be one of `"draft"`, `"internal"`, or
+    /// `"public"`. The caller owns this validation (the API layer enforces it); this
+    /// method `debug_assert!`s the constraint as defense-in-depth.
     pub async fn search_objects(
         &self,
         query: &str,
@@ -185,7 +191,14 @@ impl SearchClient {
     ) -> Result<SearchResults, SearchError> {
         let index = self.client.index(&self.index_uid);
 
-        let filter = visibility.map(|v| format!("visibility = \"{v}\""));
+        let filter = visibility.map(|v| {
+            debug_assert!(
+                matches!(v, "draft" | "internal" | "public"),
+                "visibility filter must be a known value; got {v:?}"
+            );
+
+            format!("visibility = \"{v}\"")
+        });
         let highlight: &[&str] = &["object_name", "brief_description", "fields_text"];
         let crop: &[(&str, Option<usize>)] = &[("brief_description", None), ("fields_text", None)];
 
@@ -196,6 +209,7 @@ impl SearchClient {
             .with_limit(limit)
             .with_attributes_to_highlight(Selectors::Some(highlight))
             .with_attributes_to_crop(Selectors::Some(crop))
+            // ~20 words gives enough catalogue-description context around a match.
             .with_crop_length(20)
             .with_highlight_pre_tag(HL_PRE)
             .with_highlight_post_tag(HL_POST);
@@ -226,6 +240,8 @@ impl SearchClient {
 
         Ok(SearchResults {
             hits,
+            // estimated_total_hits is always present for offset/limit pagination;
+            // None only under page-based mode, which we don't use.
             estimated_total: results.estimated_total_hits.unwrap_or(0),
         })
     }