fix: replace unreachable!() with proper error returns for DeepSeek providers (#835)

Three `unreachable!()` calls panicked if `save_api_key_for` or the API key
apply path ever received a DeepSeek/DeepseekCN variant past the early-return
guard. Replace them with explicit `Err` returns so a future refactor that
breaks the guard produces a recoverable error instead of a crash.

crates/tui/src/config.rs

@@ -2528,7 +2528,11 @@ pub fn save_api_key_for(provider: ApiProvider, api_key: &str) -> Result<PathBuf>
     ensure_parent_dir(&config_path)?;
 
     let table_name = match provider {
-        ApiProvider::Deepseek | ApiProvider::DeepseekCN => unreachable!(),
+        ApiProvider::Deepseek | ApiProvider::DeepseekCN => {
+            return Err(anyhow::anyhow!(
+                "save_api_key_for: DeepSeek variants must use the root api_key field, not provider-specific storage"
+            ));
+        }
         ApiProvider::NvidiaNim => "providers.nvidia_nim",
         ApiProvider::Openrouter => "providers.openrouter",
         ApiProvider::Novita => "providers.novita",
@@ -2556,7 +2560,11 @@ pub fn save_api_key_for(provider: ApiProvider, api_key: &str) -> Result<PathBuf>
         .as_table_mut()
         .context("`providers` must be a table.")?;
     let key_inside = match provider {
-        ApiProvider::Deepseek | ApiProvider::DeepseekCN => unreachable!(),
+        ApiProvider::Deepseek | ApiProvider::DeepseekCN => {
+            return Err(anyhow::anyhow!(
+                "save_api_key_for: DeepSeek variants must use the root api_key field, not provider-specific storage"
+            ));
+        }
         ApiProvider::NvidiaNim => "nvidia_nim",
         ApiProvider::Openrouter => "openrouter",
         ApiProvider::Novita => "novita",

crates/tui/src/tui/ui.rs

@@ -5520,7 +5520,10 @@ async fn apply_provider_picker_api_key(
             .providers
             .get_or_insert_with(ProvidersConfig::default);
         let entry: &mut ProviderConfig = match provider {
-            ApiProvider::Deepseek | ApiProvider::DeepseekCN => unreachable!(),
+            ApiProvider::Deepseek | ApiProvider::DeepseekCN => {
+                // Guarded by the outer `if` above; safety net against refactors.
+                return;
+            }
             ApiProvider::NvidiaNim => &mut providers.nvidia_nim,
             ApiProvider::Openrouter => &mut providers.openrouter,
             ApiProvider::Novita => &mut providers.novita,