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/language_model_selector/src/language_model_selector.rs

@@ -759,6 +759,10 @@ mod tests {
             false
         }
 
+        fn supports_images(&self) -> bool {
+            false
+        }
+
         fn telemetry_id(&self) -> String {
             format!("{}/{}", self.provider_id.0, self.name.0)
         }