fix: handle tool calls with empty arguments

A tool call that carries no arguments streams no argument text, so
`arguments_text` is empty and `from_str("")` fails with `EOF while parsing
a value`, aborting the whole run. The model cycle already guards this, but
two other consumers did not:

- `ConversationOutput` re-parses the streamed text on `ToolCallEnd`; and
- `create_tool_round` stored the empty text verbatim in the
  `arguments_json` column, so re-loading the round (`commit_tool_result`
  and the round loader) then failed on `from_str("")`.

Treat empty argument text as an empty object in the output projection, and
persist `{}` for it so the `arguments_json` column always holds valid JSON.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/src/cursor/conversation/output.rs

@@ -343,7 +343,14 @@ impl ConversationOutput {
                         let call = calls.get_mut(&index).ok_or_else(|| {
                             Error::Protocol(format!("unknown completed tool index: {index}"))
                         })?;
-                        call.arguments = serde_json::from_str(&call.arguments_text)?;
+                        // A tool call with no arguments streams no argument text.
+                        // Treat empty text as an empty object, matching the model
+                        // cycle, instead of failing the run on `from_str("")`.
+                        call.arguments = if call.arguments_text.trim().is_empty() {
+                            serde_json::json!({})
+                        } else {
+                            serde_json::from_str(&call.arguments_text)?
+                        };
                     }
                     RunEvent::Usage(usage) => {
                         if !self.context.compacting {

server/src/store/tool_rounds.rs

@@ -101,7 +101,14 @@ impl Store {
             .bind(&call.call_id)
             .bind(&call.model_call_id)
             .bind(&call.name)
-            .bind(&call.arguments_text)
+            // A no-argument tool call streams no argument text; persist it as an
+            // empty object so the `arguments_json` column always holds valid JSON
+            // and can be re-parsed on load.
+            .bind(if call.arguments_text.trim().is_empty() {
+                "{}"
+            } else {
+                call.arguments_text.as_str()
+            })
             .execute(&mut *tx)
             .await?;
         }