agent: Introduce ModelUsageContext (#32076)

This PR is a refactor of the existing `ModelType` in
`agent_model_selector`.

In #31848 we also need to know which context we are operating in, to
check if the configured model has image support.
In order to deduplicate the logic needed, I introduced a new type called
`ModelUsageContext` which can be used throughout the agent crate


Release Notes:

- N/A

crates/agent/src/agent.rs

@@ -33,9 +33,11 @@ use assistant_slash_command::SlashCommandRegistry;
 use client::Client;
 use feature_flags::FeatureFlagAppExt as _;
 use fs::Fs;
-use gpui::{App, actions, impl_actions};
+use gpui::{App, Entity, actions, impl_actions};
 use language::LanguageRegistry;
-use language_model::{LanguageModelId, LanguageModelProviderId, LanguageModelRegistry};
+use language_model::{
+    ConfiguredModel, LanguageModelId, LanguageModelProviderId, LanguageModelRegistry,
+};
 use prompt_store::PromptBuilder;
 use schemars::JsonSchema;
 use serde::Deserialize;
@@ -115,6 +117,23 @@ impl ManageProfiles {
 
 impl_actions!(agent, [NewThread, ManageProfiles]);
 
+#[derive(Clone)]
+pub(crate) enum ModelUsageContext {
+    Thread(Entity<Thread>),
+    InlineAssistant,
+}
+
+impl ModelUsageContext {
+    pub fn configured_model(&self, cx: &App) -> Option<ConfiguredModel> {
+        match self {
+            Self::Thread(thread) => thread.read(cx).configured_model(),
+            Self::InlineAssistant => {
+                LanguageModelRegistry::read_global(cx).inline_assistant_model()
+            }
+        }
+    }
+}
+
 /// Initializes the `agent` crate.
 pub fn init(
     fs: Arc<dyn Fs>,