Allow models.json to override built-in providers (#406)

* Allow models.json to override built-in providers

When a provider is defined in models.json with the same name as a
built-in provider (e.g., 'anthropic', 'google'), the built-in models
for that provider are completely replaced by the custom definition.

This enables users to:
- Use custom base URLs (proxies, self-hosted endpoints)
- Define a subset of models they want available
- Customize model configurations for built-in providers

Example usage in ~/.pi/agent/models.json:
{
  "providers": {
    "anthropic": {
      "baseUrl": "https://my-proxy.example.com/v1",
      "apiKey": "ANTHROPIC_API_KEY",
      "api": "anthropic-messages",
      "models": [...]
    }
  }
}

* Refactor model-registry for readability

- Extract CustomModelsResult type and emptyCustomModelsResult helper
- Extract loadBuiltInModels method with clear skip logic
- Simplify loadModels with destructuring and ternary
- Reduce repetition in error handling paths

* Refactor model-registry tests for readability

- Extract providerConfig() helper to hide irrelevant model fields
- Extract writeModelsJson() helper for file writing
- Extract getModelsForProvider() helper for filtering
- Move modelsJsonPath to beforeEach

Reduces test file from 262 to 130 lines while maintaining same coverage.

packages/coding-agent/src/core/model-registry.ts

@@ -74,6 +74,17 @@ const ModelsConfigSchema = Type.Object({
 
 type ModelsConfig = Static<typeof ModelsConfigSchema>;
 
+/** Result of loading custom models from models.json */
+interface CustomModelsResult {
+	models: Model<Api>[];
+	providers: Set<string>;
+	error: string | undefined;
+}
+
+function emptyCustomModelsResult(error?: string): CustomModelsResult {
+	return { models: [], providers: new Set(), error };
+}
+
 /**
  * Resolve an API key config value to an actual key.
  * Checks environment variable first, then treats as literal.
@@ -126,25 +137,17 @@ export class ModelRegistry {
 	}
 
 	private loadModels(): void {
-		// Load built-in models
-		const builtInModels: Model<Api>[] = [];
-		for (const provider of getProviders()) {
-			const providerModels = getModels(provider as KnownProvider);
-			builtInModels.push(...(providerModels as Model<Api>[]));
-		}
-
-		// Load custom models from models.json (if path provided)
-		let customModels: Model<Api>[] = [];
-		if (this.modelsJsonPath) {
-			const result = this.loadCustomModels(this.modelsJsonPath);
-			if (result.error) {
-				this.loadError = result.error;
-				// Keep built-in models even if custom models failed to load
-			} else {
-				customModels = result.models;
-			}
+		// Load custom models from models.json first (to know which providers to skip)
+		const { models: customModels, providers: customProviders, error } = this.modelsJsonPath
+			? this.loadCustomModels(this.modelsJsonPath)
+			: emptyCustomModelsResult();
+
+		if (error) {
+			this.loadError = error;
+			// Keep built-in models even if custom models failed to load
 		}
 
+		const builtInModels = this.loadBuiltInModels(customProviders);
 		const combined = [...builtInModels, ...customModels];
 
 		// Update github-copilot base URL based on OAuth credentials
@@ -160,9 +163,16 @@ export class ModelRegistry {
 		}
 	}
 
-	private loadCustomModels(modelsJsonPath: string): { models: Model<Api>[]; error: string | undefined } {
+	/** Load built-in models, skipping providers that are overridden in models.json */
+	private loadBuiltInModels(skipProviders: Set<string>): Model<Api>[] {
+		return getProviders()
+			.filter((provider) => !skipProviders.has(provider))
+			.flatMap((provider) => getModels(provider as KnownProvider) as Model<Api>[]);
+	}
+
+	private loadCustomModels(modelsJsonPath: string): CustomModelsResult {
 		if (!existsSync(modelsJsonPath)) {
-			return { models: [], error: undefined };
+			return emptyCustomModelsResult();
 		}
 
 		try {
@@ -176,28 +186,22 @@ export class ModelRegistry {
 				const errors =
 					validate.errors?.map((e: any) => `  - ${e.instancePath || "root"}: ${e.message}`).join("\n") ||
 					"Unknown schema error";
-				return {
-					models: [],
-					error: `Invalid models.json schema:\n${errors}\n\nFile: ${modelsJsonPath}`,
-				};
+				return emptyCustomModelsResult(`Invalid models.json schema:\n${errors}\n\nFile: ${modelsJsonPath}`);
 			}
 
 			// Additional validation
 			this.validateConfig(config);
 
-			// Parse models
-			return { models: this.parseModels(config), error: undefined };
+			// Parse models and collect provider names
+			const providers = new Set(Object.keys(config.providers));
+			return { models: this.parseModels(config), providers, error: undefined };
 		} catch (error) {
 			if (error instanceof SyntaxError) {
-				return {
-					models: [],
-					error: `Failed to parse models.json: ${error.message}\n\nFile: ${modelsJsonPath}`,
-				};
+				return emptyCustomModelsResult(`Failed to parse models.json: ${error.message}\n\nFile: ${modelsJsonPath}`);
 			}
-			return {
-				models: [],
-				error: `Failed to load models.json: ${error instanceof Error ? error.message : error}\n\nFile: ${modelsJsonPath}`,
-			};
+			return emptyCustomModelsResult(
+				`Failed to load models.json: ${error instanceof Error ? error.message : error}\n\nFile: ${modelsJsonPath}`,
+			);
 		}
 	}