agent: Handle tool use without text (#28030)

### Context 

The Anthropic API fails if a request message contains a tool use and no
`Text` segments or it only contains empty `Text` segments. These are
cases that the model itself produces, but the API doesn't support
sending them back.

#27917 fixed this by appending "Using tool..." in the thread's message,
but this causes the actual conversation to include it, so it would
appear in the UI (we would actually display a gap because we never
rendered its markdown, but "Using tool..." would show up when the thread
was restored).

### Solution

We'll now only append this placeholder when we build the request, so the
API still sees it, but the UI/Thread doesn't.

Another issue we found is that the model starts mimicking these
placeholders in later tool uses which is undesirable. So unfortunately,
we had to add logic to filter them out.

Release Notes:

- agent: Improved rendering of tool uses without text

---------

Co-authored-by: Bennet <bennet@zed.dev>

crates/agent/src/tool_use.rs

@@ -43,6 +43,8 @@ pub struct ToolUseState {
     pending_tool_uses_by_id: HashMap<LanguageModelToolUseId, PendingToolUse>,
 }
 
+pub const USING_TOOL_MARKER: &str = "<using_tool>";
+
 impl ToolUseState {
     pub fn new(tools: Arc<ToolWorkingSet>) -> Self {
         Self {
@@ -357,8 +359,28 @@ impl ToolUseState {
         request_message: &mut LanguageModelRequestMessage,
     ) {
         if let Some(tool_uses) = self.tool_uses_by_assistant_message.get(&message_id) {
+            let mut found_tool_use = false;
+
             for tool_use in tool_uses {
                 if self.tool_results.contains_key(&tool_use.id) {
+                    if !found_tool_use {
+                        // The API fails if a message contains a tool use without any (non-whitespace) text around it
+                        match request_message.content.last_mut() {
+                            Some(MessageContent::Text(txt)) => {
+                                if txt.is_empty() {
+                                    txt.push_str(USING_TOOL_MARKER);
+                                }
+                            }
+                            None | Some(_) => {
+                                request_message
+                                    .content
+                                    .push(MessageContent::Text(USING_TOOL_MARKER.into()));
+                            }
+                        };
+                    }
+
+                    found_tool_use = true;
+
                     // Do not send tool uses until they are completed
                     request_message
                         .content