Fix OpenCode model discovery when npm is omitted (#65872)

Fixes #65710.

OpenCode's catalog includes per-model provider overrides containing only
an API URL. Make the per-model npm transport optional and inherit the
parent provider's transport when it is absent, while preserving explicit
per-model overrides.

Add regression coverage for transport inheritance, API-only provider
overrides, and explicit transport overrides for both Zen and Go.

Release Notes:

- Fixed OpenCode models failing to load with a missing field `npm`
error.

crates/language_models/src/provider/opencode.rs

@@ -1063,7 +1063,7 @@ struct ModelLimits {
 
 #[derive(Deserialize)]
 struct ModelTransport {
-    npm: String,
+    npm: Option<String>,
 }
 
 #[derive(Deserialize)]
@@ -1117,7 +1117,7 @@ fn parse_discovered_models(
             let transport = metadata
                 .provider
                 .as_ref()
-                .map(|provider| provider.npm.as_str())
+                .and_then(|provider| provider.npm.as_deref())
                 .or(provider.npm.as_deref());
             let protocol = transport.and_then(protocol_for_transport)?;
             let reasoning_effort_levels = metadata
@@ -1833,6 +1833,50 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn test_discovery_inherits_transport_unless_model_overrides_it() -> Result<()> {
+        for (model_provider, expected_protocol) in [
+            (serde_json::Value::Null, ApiProtocol::OpenAiChat),
+            (
+                serde_json::json!({"api": "https://opencode.ai/inference/openai/v1"}),
+                ApiProtocol::OpenAiChat,
+            ),
+            (
+                serde_json::json!({"npm": "@ai-sdk/anthropic"}),
+                ApiProtocol::Anthropic,
+            ),
+        ] {
+            let provider = serde_json::json!({
+                "npm": "@ai-sdk/openai-compatible",
+                "models": {
+                    "model": {
+                        "id": "model",
+                        "name": "Model",
+                        "tool_call": true,
+                        "modalities": {"output": ["text"]},
+                        "limit": {"context": 1000},
+                        "provider": model_provider,
+                    },
+                },
+            });
+            let catalog = serde_json::json!({
+                "opencode": provider,
+                "opencode-go": provider,
+            })
+            .to_string();
+            for subscription in [OpenCodeSubscription::Zen, OpenCodeSubscription::Go] {
+                let models = parse_discovered_models(&catalog, subscription)?;
+                assert_eq!(models.len(), 1, "{subscription:?}: {model_provider}");
+                assert_eq!(
+                    models[0].model.protocol(),
+                    expected_protocol,
+                    "{subscription:?}: {model_provider}",
+                );
+            }
+        }
+        Ok(())
+    }
+
     #[test]
     fn test_parse_discovered_models_uses_catalog_entries_directly() {
         let models = parse_discovered_models(