fix(gemini): resolve 400 Bad Request by sanitizing thought_signature and improving tool name resolution

This commit fixes the Gemini API 'Invalid value at thought_signature' error by ensuring synthetic 'call_' IDs are not passed into the TYPE_BYTES field. It also adds a pre-pass to correctly resolve function names from tool call IDs for tool responses.

src/providers/gemini.rs

@@ -222,6 +222,16 @@ impl GeminiProvider {
         let mut contents: Vec<GeminiContent> = Vec::new();
         let mut system_parts = Vec::new();
 
+        // PRE-PASS: Build tool_id -> function_name mapping for tool responses
+        let mut tool_id_to_name = std::collections::HashMap::new();
+        for msg in &messages {
+            if let Some(tool_calls) = &msg.tool_calls {
+                for tc in tool_calls {
+                    tool_id_to_name.insert(tc.id.clone(), tc.function.name.clone());
+                }
+            }
+        }
+
         for msg in messages {
             if msg.role == "system" {
                 for part in msg.content {
@@ -261,7 +271,14 @@ impl GeminiProvider {
                     })
                     .unwrap_or_default();
 
-                let name = msg.name.clone().or_else(|| msg.tool_call_id.clone()).unwrap_or_else(|| "unknown_function".to_string());
+                // RESOLVE: Use msg.name if present, otherwise look up by tool_call_id
+                let name = msg.name.clone()
+                    .or_else(|| {
+                        msg.tool_call_id.as_ref()
+                            .and_then(|id| tool_id_to_name.get(id).cloned())
+                    })
+                    .or_else(|| msg.tool_call_id.clone())
+                    .unwrap_or_else(|| "unknown_function".to_string());
                 
                 // Gemini API requires 'response' to be a JSON object (google.protobuf.Struct).
                 // If it is an array or primitive, wrap it in an object.
@@ -322,10 +339,13 @@ impl GeminiProvider {
                         let args = serde_json::from_str::<Value>(&tc.function.arguments)
                             .unwrap_or_else(|_| serde_json::json!({}));
                         
-                        // RESTORE: Use tc.id as thought_signature.
+                        // RESTORE: Only use tc.id as thought_signature if it's NOT a synthetic ID.
-                        // Gemini 3 models require this field for any function call in the history.
+                        // Synthetic IDs (starting with 'call_') cause 400 errors as they are not valid Base64 for the TYPE_BYTES field.
-                        // We include it regardless of format to ensure the model has context.
+                        let thought_signature = if tc.id.starts_with("call_") {
-                        let thought_signature = Some(tc.id.clone());
+                            None
+                        } else {
+                            Some(tc.id.clone())
+                        };
 
                         parts.push(GeminiPart {
                             text: None,