Allow tools to read unsaved buffers (#26987)

If the tool asks to read a path, we don't need to verify whether that
path exists on disk; an unsaved buffer with that path is fine.

Release Notes:

- N/A

crates/assistant_tools/src/read_file_tool.rs

@@ -78,12 +78,8 @@ impl Tool for ReadFileTool {
                 .await?;
 
             let result = buffer.read_with(cx, |buffer, _cx| {
-                if buffer
-                    .file()
-                    .map_or(false, |file| file.disk_state().exists())
-                {
                 let text = buffer.text();
-                    let string = if input.start_line.is_some() || input.end_line.is_some() {
+                if input.start_line.is_some() || input.end_line.is_some() {
                     let start = input.start_line.unwrap_or(1);
                     let lines = text.split('\n').skip(start - 1);
                     if let Some(end) = input.end_line {
@@ -94,13 +90,8 @@ impl Tool for ReadFileTool {
                     }
                 } else {
                     text
-                    };
-
-                    Ok(string)
-                } else {
-                    Err(anyhow!("File does not exist"))
                 }
-            })??;
+            })?;
 
             action_log.update(cx, |log, cx| {
                 log.buffer_read(buffer, cx);