fix(gemini): strictly enforce alternating roles and improve message merging

- Merge all consecutive messages with the same role into a single GeminiContent object.
- Ensure the first message is always 'user' by prepending a placeholder if necessary.
- Add final check for empty contents to prevent sending malformed requests.
- This addresses strict role-sequence requirements in Gemini 2.0/3.0 models.

src/providers/gemini.rs

@@ -216,7 +216,7 @@ impl GeminiProvider {
 
             let role = match msg.role.as_str() {
                 "assistant" => "model".to_string(),
-                "tool" => "user".to_string(), // Tool results are technically from the user side in Gemini
+                "tool" => "user".to_string(), // Tool results are user-side in Gemini
                 _ => "user".to_string(),
             };
 
@@ -233,7 +233,6 @@ impl GeminiProvider {
                     })
                     .unwrap_or_default();
 
-                // Gemini function response MUST have a name. Fallback to tool_call_id if name is missing.
                 let name = msg.name.clone().or_else(|| msg.tool_call_id.clone()).unwrap_or_else(|| "unknown_function".to_string());
                 let response_value = serde_json::from_str::<Value>(&text_content)
                     .unwrap_or_else(|_| serde_json::json!({ "result": text_content }));
@@ -250,7 +249,6 @@ impl GeminiProvider {
             } else if msg.role == "assistant" && msg.tool_calls.is_some() {
                 // Assistant messages with tool_calls
                 if let Some(tool_calls) = &msg.tool_calls {
-                    // Include text content if present
                     for p in &msg.content {
                         if let ContentPart::Text { text } = p {
                             if !text.trim().is_empty() {
@@ -316,7 +314,8 @@ impl GeminiProvider {
                 continue;
             }
 
-            // Merge with previous message if role matches
+            // STRATEGY: Strictly enforce alternating roles.
+            // If current message has the same role as the last one, merge their parts.
             if let Some(last_content) = contents.last_mut() {
                 if last_content.role.as_ref() == Some(&role) {
                     last_content.parts.extend(parts);
@@ -331,7 +330,6 @@ impl GeminiProvider {
         }
 
         // Gemini requires the first message to be from "user".
-        // If it starts with "model", we prepend a placeholder user message.
         if let Some(first) = contents.first() {
             if first.role.as_deref() == Some("model") {
                 contents.insert(0, GeminiContent {
@@ -346,6 +344,12 @@ impl GeminiProvider {
             }
         }
 
+        // Final check: ensure we don't have empty contents after filtering.
+        // If the last message was merged or filtered, we might have an empty array.
+        if contents.is_empty() && system_parts.is_empty() {
+            return Err(AppError::ProviderError("No valid content parts after filtering".to_string()));
+        }
+
         let system_instruction = if !system_parts.is_empty() {
             Some(GeminiContent {
                 parts: system_parts,