vercel: Use proper model identifiers and add image support (#33377)

Follow up to previous PRs:
- Return `true` in `supports_images` - v0 supports images already
- Rename model id to match the exact version of the model `v0-1.5-md`
(For now we do not expose `sm`/`lg` variants since they seem not to be
available via the API)
- Provide autocompletion in settings for using `vercel` as a `provider`

Release Notes:

- N/A

crates/agent_settings/src/agent_settings.rs

@@ -734,6 +734,7 @@ impl JsonSchema for LanguageModelProviderSetting {
                 "deepseek".into(),
                 "openrouter".into(),
                 "mistral".into(),
+                "vercel".into(),
             ]),
             ..Default::default()
         }