Regenerate embeddings more often:

- When `regenerate` was previously `false` and became `true`
- When rendering the old version of the docs failed

crates/milli/src/update/new/extract/vectors/mod.rs

@@ -141,6 +141,10 @@ impl<'extractor> Extractor<'extractor> for EmbeddingExtractor<'_, '_> {
                                     context.new_fields_ids_map,
                                     &context.doc_alloc,
                                 )?;
+                                let must_regenerate = if !old_vectors.regenerate {
+                                    // we just enabled `regenerate`
+                                    true
+                                } else {
                                     let old_rendered = prompt.render_document(
                                         update.external_document_id(),
                                         update.merged(
@@ -150,8 +154,18 @@ impl<'extractor> Extractor<'extractor> for EmbeddingExtractor<'_, '_> {
                                         )?,
                                         context.new_fields_ids_map,
                                         &context.doc_alloc,
-                                )?;
+                                    );
-                                if new_rendered != old_rendered {
+
+                                    if let Ok(old_rendered) = old_rendered {
+                                        // must regenerate if the rendered changed
+                                        new_rendered != old_rendered
+                                    } else {
+                                        // cannot check previous rendered, better regenerate
+                                        true
+                                    }
+                                };
+
+                                if must_regenerate {
                                     chunks.set_autogenerated(
                                         update.docid(),
                                         update.external_document_id(),