agent: Show warning for image context pill if model doesn't support images (#31848)

Closes #31781

Currently we don't any warning or error if the image is not supported by
the current model in selected in the agent panel which leads for users
to think it's supported as there is no visual feedback provided by zed.
This PR adds a warning on image context pill to show warning when the
model doesn't support it.

| Before | After |
|--------|-------|
| <img width="374" alt="image"
src="https://github.com/user-attachments/assets/da659fb6-d5da-4c53-8878-7a1c4553f168"
/> | <img width="442" alt="image"
src="https://github.com/user-attachments/assets/0f23d184-6095-47e2-8f2b-0eac64a0942e"
/> |

Release Notes:

- Show warning for image context pill in agent panel when selected model
doesn't support images.

---------

Signed-off-by: Umesh Yadav <git@umesh.dev>
Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>

crates/agent/src/message_editor.rs

@@ -169,6 +169,7 @@ impl MessageEditor {
                 Some(text_thread_store.clone()),
                 context_picker_menu_handle.clone(),
                 SuggestContextKind::File,
+                ModelUsageContext::Thread(thread.clone()),
                 window,
                 cx,
             )