fix(coding-agent): custom models for built-in providers and list-models error surfacing (#3072)

* fix(coding-agent): surface models.json load errors on stderr in --list-models

When models.json has validation errors, --list-models silently
discarded custom models and overrides without any user-visible
feedback. Now prints the error to stderr via chalk.yellow warning
before listing available models.

* fix(coding-agent): allow custom models for built-in providers in models.json

Built-in providers (openrouter, anthropic, etc.) already have api and
baseUrl on every model, and auth comes from env vars / auth storage.
Relax validation so custom models under built-in providers don't need
redundant baseUrl, apiKey, or api fields. Inherit them from the first
built-in model for that provider.

fixes #2921

---------

Co-authored-by: Mario Zechner <badlogicgames@gmail.com>

packages/coding-agent/src/cli/list-models.ts

@@ -4,6 +4,7 @@
 
 import type { Api, Model } from "@mariozechner/pi-ai";
 import { fuzzyFilter } from "@mariozechner/pi-tui";
+import chalk from "chalk";
 import type { ModelRegistry } from "../core/model-registry.js";
 
 /**
@@ -25,6 +26,11 @@ function formatTokenCount(count: number): string {
  * List available models, optionally filtered by search pattern
  */
 export async function listModels(modelRegistry: ModelRegistry, searchPattern?: string): Promise<void> {
+	const loadError = modelRegistry.getError();
+	if (loadError) {
+		console.error(chalk.yellow(`Warning: errors loading models.json:\n${loadError}`));
+	}
+
 	const models = modelRegistry.getAvailable();
 
 	if (models.length === 0) {