Have read_file support images (#30435)

This is very basic support for them. There are a number of other TODOs
before this is really a first-class supported feature, so not adding any
release notes for it; for now, this PR just makes it so that if
read_file tries to read a PNG (which has come up in practice), it at
least correctly sends it to Anthropic instead of messing up.

This also lays the groundwork for future PRs for more first-class
support for images in tool calls across more image file formats and LLM
providers.

Release Notes:

- N/A

---------

Co-authored-by: Agus Zubiaga <hi@aguz.me>
Co-authored-by: Agus Zubiaga <agus@zed.dev>

crates/assistant_tools/src/grep_tool.rs

@@ -752,9 +752,9 @@ mod tests {
         match task.output.await {
             Ok(result) => {
                 if cfg!(windows) {
-                    result.content.replace("root\\", "root/")
+                    result.content.as_str().unwrap().replace("root\\", "root/")
                 } else {
-                    result.content
+                    result.content.as_str().unwrap().to_string()
                 }
             }
             Err(e) => panic!("Failed to run grep tool: {}", e),