agent: Model name context (#41490)

Closes #41478

Release Notes:

- Fixed #41478

<img width="459" height="916" alt="Screenshot 2025-10-29 at 1 31 26 PM"
src="https://github.com/user-attachments/assets/1d5b9fdf-9800-44e4-bdd5-f0964f93625f"
/>

> caused by using haiku 4.5 from the anthropic provider and then
swapping to sonnet 3.7 through zed, doing this does mess with prompt
caching but a model swap already invalidates that so it shouldn't have
any cost impact on end users

crates/agent/src/thread.rs

@@ -1928,6 +1928,7 @@ impl Thread {
         let system_prompt = SystemPromptTemplate {
             project: self.project_context.read(cx),
             available_tools,
+            model_name: self.model.as_ref().map(|m| m.name().0.to_string()),
         }
         .render(&self.templates)
         .context("failed to build system prompt")