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"
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> 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/edit_agent/evals.rs

@@ -1581,6 +1581,7 @@ impl EditAgentTest {
             let template = crate::SystemPromptTemplate {
                 project: &project_context,
                 available_tools: tool_names,
+                model_name: None,
             };
             let templates = Templates::new();
             template.render(&templates).unwrap()