fix: sanitize tool call IDs for OpenAI compatibility

OpenAI has a strict 40-character limit for tool call IDs. Long IDs (like
Gemini's 56-char thought signatures) are now deterministically truncated
to 40 characters when sending history back to OpenAI-compatible providers.

src/providers/helpers.rs

@@ -29,7 +29,14 @@ pub async fn messages_to_openai_json(messages: &[UnifiedMessage]) -> Result<Vec<
                 "content": text_content
             });
             if let Some(tool_call_id) = &m.tool_call_id {
-                msg["tool_call_id"] = serde_json::json!(tool_call_id);
+                // OpenAI and others have a 40-char limit for tool_call_id.
+                // Gemini signatures (56 chars) must be shortened for compatibility.
+                let id = if tool_call_id.len() > 40 {
+                    &tool_call_id[..40]
+                } else {
+                    tool_call_id
+                };
+                msg["tool_call_id"] = serde_json::json!(id);
             }
             if let Some(name) = &m.name {
                 msg["name"] = serde_json::json!(name);
@@ -60,14 +67,28 @@ pub async fn messages_to_openai_json(messages: &[UnifiedMessage]) -> Result<Vec<
 
         let mut msg = serde_json::json!({ "role": m.role });
 
+        // Include reasoning_content if present (DeepSeek R1/reasoner requires this in history)
+        if let Some(reasoning) = &m.reasoning_content {
+            msg["reasoning_content"] = serde_json::json!(reasoning);
+        }
+
         // For assistant messages with tool_calls, content can be null
         if let Some(tool_calls) = &m.tool_calls {
+            // Sanitize tool call IDs for OpenAI compatibility (max 40 chars)
+            let sanitized_calls: Vec<_> = tool_calls.iter().map(|tc| {
+                let mut sanitized = tc.clone();
+                if sanitized.id.len() > 40 {
+                    sanitized.id = sanitized.id[..40].to_string();
+                }
+                sanitized
+            }).collect();
+
             if parts.is_empty() {
                 msg["content"] = serde_json::Value::Null;
             } else {
                 msg["content"] = serde_json::json!(parts);
             }
-            msg["tool_calls"] = serde_json::json!(tool_calls);
+            msg["tool_calls"] = serde_json::json!(sanitized_calls);
         } else {
             msg["content"] = serde_json::json!(parts);
         }
@@ -109,7 +130,13 @@ pub async fn messages_to_openai_json_text_only(
                 "content": text_content
             });
             if let Some(tool_call_id) = &m.tool_call_id {
-                msg["tool_call_id"] = serde_json::json!(tool_call_id);
+                // OpenAI and others have a 40-char limit for tool_call_id.
+                let id = if tool_call_id.len() > 40 {
+                    &tool_call_id[..40]
+                } else {
+                    tool_call_id
+                };
+                msg["tool_call_id"] = serde_json::json!(id);
             }
             if let Some(name) = &m.name {
                 msg["name"] = serde_json::json!(name);
@@ -133,14 +160,28 @@ pub async fn messages_to_openai_json_text_only(
 
         let mut msg = serde_json::json!({ "role": m.role });
 
+        // Include reasoning_content if present (DeepSeek R1/reasoner requires this in history)
+        if let Some(reasoning) = &m.reasoning_content {
+            msg["reasoning_content"] = serde_json::json!(reasoning);
+        }
+
         // For assistant messages with tool_calls, content can be null
         if let Some(tool_calls) = &m.tool_calls {
+            // Sanitize tool call IDs for OpenAI compatibility (max 40 chars)
+            let sanitized_calls: Vec<_> = tool_calls.iter().map(|tc| {
+                let mut sanitized = tc.clone();
+                if sanitized.id.len() > 40 {
+                    sanitized.id = sanitized.id[..40].to_string();
+                }
+                sanitized
+            }).collect();
+
             if parts.is_empty() {
                 msg["content"] = serde_json::Value::Null;
             } else {
                 msg["content"] = serde_json::json!(parts);
             }
-            msg["tool_calls"] = serde_json::json!(tool_calls);
+            msg["tool_calls"] = serde_json::json!(sanitized_calls);
         } else {
             msg["content"] = serde_json::json!(parts);
         }