fix: comprehensive cross-provider tool calling and history fixes

- Fix DeepSeek R1 (reasoner) 400 errors by ensuring assistant messages with
  tool_calls in history always have non-null 'content' and 'reasoning_content'.
- Implement deterministic tool call ID truncation (max 40 chars) for OpenAI
  compatibility (fixes errors when history contains long Gemini signatures).
- Automatic transition from 'max_tokens' to 'max_completion_tokens' for newer
  OpenAI models (o1, o3, gpt-5-nano).
- Added 'reasoning' and 'thought' aliases to reasoning_content for robust
  deserialization from various clients.

src/providers/helpers.rs

@@ -72,7 +72,7 @@ pub async fn messages_to_openai_json(messages: &[UnifiedMessage]) -> Result<Vec<
             msg["reasoning_content"] = serde_json::json!(reasoning);
         }
 
-        // For assistant messages with tool_calls, content can be null
+        // For assistant messages with tool_calls, content can be empty string
         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| {
@@ -84,7 +84,7 @@ pub async fn messages_to_openai_json(messages: &[UnifiedMessage]) -> Result<Vec<
             }).collect();
 
             if parts.is_empty() {
-                msg["content"] = serde_json::Value::Null;
+                msg["content"] = serde_json::json!("");
             } else {
                 msg["content"] = serde_json::json!(parts);
             }
@@ -165,7 +165,7 @@ pub async fn messages_to_openai_json_text_only(
             msg["reasoning_content"] = serde_json::json!(reasoning);
         }
 
-        // For assistant messages with tool_calls, content can be null
+        // For assistant messages with tool_calls, content can be empty string
         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| {
@@ -177,7 +177,7 @@ pub async fn messages_to_openai_json_text_only(
             }).collect();
 
             if parts.is_empty() {
-                msg["content"] = serde_json::Value::Null;
+                msg["content"] = serde_json::json!("");
             } else {
                 msg["content"] = serde_json::json!(parts);
             }