style: apply clippy suggestions for code quality improvements

- Replace raw string literal `r#"[1, 2, 3]"#` with simpler `r"[1, 2, 3]"` in extract_json_direct_array test
- Remove redundant `chrono::` prefix from `Utc::now()` call in metadata_filter_not_empty_with_timestamp test
- Replace manual dot product calculation with `mul_add` for better numerical precision in cosine_similarity_arbitrary test

Author: gitctrlx

meme/src/llm/client.rs

@@ -352,7 +352,7 @@ mod tests {
 
     #[test]
     fn extract_json_direct_array() {
-        let v = extract_json_from_text(r#"[1, 2, 3]"#).unwrap();
+        let v = extract_json_from_text(r"[1, 2, 3]").unwrap();
         assert_eq!(v.as_array().unwrap().len(), 3);
     }
 

meme/src/model/entry.rs

@@ -131,7 +131,7 @@ mod tests {
 
     #[test]
     fn metadata_filter_not_empty_with_timestamp() {
-        let now = chrono::Utc::now();
+        let now = Utc::now();
         let f = MetadataFilter {
             timestamp_range: Some((now, now)),
             ..Default::default()

meme/src/store/vector.rs

@@ -846,7 +846,7 @@ mod tests {
     fn cosine_similarity_arbitrary() {
         let a = vec![1.0, 2.0, 3.0];
         let b = vec![4.0, 5.0, 6.0];
-        let dot = 1.0 * 4.0 + 2.0 * 5.0 + 3.0 * 6.0;
+        let dot = 3.0f64.mul_add(6.0, 2.0f64.mul_add(5.0, 1.0 * 4.0));
         let mag_a = (1.0_f64 + 4.0 + 9.0).sqrt();
         let mag_b = (16.0_f64 + 25.0 + 36.0).sqrt();
         let expected = dot / (mag_a * mag_b);