fix: use effective_model_for_budget instead of raw model in compactio… (#2437)

* fix: use effective_model_for_budget instead of raw model in compaction_config

When model is set to 'auto', self.model holds the literal string 'auto',
which gets passed to the API as the model name in compaction requests.
DeepSeek's API rejects 'auto' with HTTP 400 since it's not a recognized
model ID.

effective_model_for_budget() resolves 'auto' to the last effective model
or falls back to DEFAULT_TEXT_MODEL, ensuring compaction always sends a
concrete model name.

* test(tui): cover auto model compaction config

---------

Co-authored-by: codgo <anbiaoren@gie777.com>
Co-authored-by: Hunter B <hmbown@gmail.com>

crates/tui/src/tui/app.rs

@@ -4733,7 +4733,7 @@ impl App {
         CompactionConfig {
             enabled: self.auto_compact,
             token_threshold: self.compact_threshold,
-            model: self.model.clone(),
+            model: self.effective_model_for_budget().to_string(),
             ..Default::default()
         }
     }
@@ -5985,10 +5985,20 @@ mod tests {
 
     #[test]
     fn test_compaction_config() {
-        let app = App::new(test_options(false), &Config::default());
+        let mut app = App::new(test_options(false), &Config::default());
         let config = app.compaction_config();
         // Config should be valid (just checking it returns something)
         let _ = config.enabled;
+
+        app.auto_model = true;
+        app.model = "auto".to_string();
+        app.last_effective_model = None;
+        let config = app.compaction_config();
+        assert_eq!(config.model, DEFAULT_TEXT_MODEL);
+
+        app.last_effective_model = Some("deepseek-v4-flash".to_string());
+        let config = app.compaction_config();
+        assert_eq!(config.model, "deepseek-v4-flash");
     }
 
     #[test]