refactor: parse --json-values once as serde_json::Value, then branch

Address Gemini review: avoid parsing the JSON string twice. Now parses
once into a generic Value, then branches on whether it's a nested
array (multi-row) or flat array (single row). Also rejects non-array
JSON (objects, scalars) with a clear error.

Author: anshul-garg27

src/helpers/sheets.rs

@@ -274,16 +274,35 @@ pub struct AppendConfig {
 /// Returns a validation error if `--json-values` contains invalid JSON.
 pub fn parse_append_args(matches: &ArgMatches) -> Result<AppendConfig, GwsError> {
     let values = if let Some(json_str) = matches.get_one::<String>("json-values") {
-        // Parse JSON array of rows
-        if let Ok(parsed) = serde_json::from_str::<Vec<Vec<String>>>(json_str) {
-            parsed.into_iter().flatten().collect()
-        } else {
-            // Treat as single row JSON array
-            serde_json::from_str::<Vec<String>>(json_str).map_err(|e| {
+        // Parse once as a generic JSON Value, then branch on structure.
+        let parsed: serde_json::Value =
+            serde_json::from_str(json_str).map_err(|e| {
                 GwsError::Validation(format!(
                     "--json-values is not valid JSON: {e}. Expected a JSON array like '[\"a\",\"b\"]' or '[[\"a\",\"b\"],[\"c\",\"d\"]]'."
                 ))
-            })?
+            })?;
+
+        match parsed {
+            // Multi-row: [[...], [...]]
+            serde_json::Value::Array(ref rows)
+                if rows.first().is_some_and(|r| r.is_array()) =>
+            {
+                rows.iter()
+                    .filter_map(|row| row.as_array())
+                    .flatten()
+                    .filter_map(|v| v.as_str().map(|s| s.to_string()))
+                    .collect()
+            }
+            // Single row: [...]
+            serde_json::Value::Array(ref items) => items
+                .iter()
+                .filter_map(|v| v.as_str().map(|s| s.to_string()))
+                .collect(),
+            _ => {
+                return Err(GwsError::Validation(
+                    "--json-values must be a JSON array, not an object or scalar.".to_string(),
+                ));
+            }
         }
     } else if let Some(values_str) = matches.get_one::<String>("values") {
         values_str.split(',').map(|s| s.to_string()).collect()