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/src/language_model.rs

@@ -243,6 +243,9 @@ pub trait LanguageModel: Send + Sync {
         LanguageModelAvailability::Public
     }
 
+    /// Whether this model supports images
+    fn supports_images(&self) -> bool;
+
     /// Whether this model supports tools.
     fn supports_tools(&self) -> bool;