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/edit_file_tool.rs

@@ -5,7 +5,8 @@ use crate::{
 };
 use anyhow::{Result, anyhow};
 use assistant_tool::{
-    ActionLog, AnyToolCard, Tool, ToolCard, ToolResult, ToolResultOutput, ToolUseStatus,
+    ActionLog, AnyToolCard, Tool, ToolCard, ToolResult, ToolResultContent, ToolResultOutput,
+    ToolUseStatus,
 };
 use buffer_diff::{BufferDiff, BufferDiffSnapshot};
 use editor::{Editor, EditorMode, MultiBuffer, PathKey};
@@ -292,7 +293,10 @@ impl Tool for EditFileTool {
                 }
             } else {
                 Ok(ToolResultOutput {
-                    content: format!("Edited {}:\n\n```diff\n{}\n```", input_path, diff),
+                    content: ToolResultContent::Text(format!(
+                        "Edited {}:\n\n```diff\n{}\n```",
+                        input_path, diff
+                    )),
                     output: serde_json::to_value(output).ok(),
                 })
             }