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/anthropic/src/anthropic.rs

@@ -534,12 +534,26 @@ pub enum RequestContent {
     ToolResult {
         tool_use_id: String,
         is_error: bool,
-        content: String,
+        content: ToolResultContent,
         #[serde(skip_serializing_if = "Option::is_none")]
         cache_control: Option<CacheControl>,
     },
 }
 
+#[derive(Debug, Serialize, Deserialize)]
+#[serde(untagged)]
+pub enum ToolResultContent {
+    JustText(String),
+    Multipart(Vec<ToolResultPart>),
+}
+
+#[derive(Debug, Serialize, Deserialize)]
+#[serde(tag = "type", rename_all = "lowercase")]
+pub enum ToolResultPart {
+    Text { text: String },
+    Image { source: ImageSource },
+}
+
 #[derive(Debug, Serialize, Deserialize)]
 #[serde(tag = "type")]
 pub enum ResponseContent {