feat: add support for custom provider configuration in the user config (#537)

### What

- Add support for loading and merging custom provider configurations
from a local `providers.json` file.
- Allow users to override or extend default providers with their own
settings.

### Why

This change enables users to flexibly customize and extend provider
endpoints and API keys without modifying the codebase, making the CLI
more adaptable for various LLM backends and enterprise use cases.

### How

- Introduced `loadProvidersFromFile` and `getMergedProviders` in config
logic.
- Added/updated related tests in [tests/config.test.tsx]


### Checklist

- [x] Lint passes for changed files
- [x] Tests pass for all files
- [x] Documentation/comments updated as needed

---------

Co-authored-by: Thibault Sottiaux <tibo@openai.com>

codex-cli/tests/config.test.tsx

@@ -5,6 +5,7 @@ import { AutoApprovalMode } from "../src/utils/auto-approval-mode.js";
 import { tmpdir } from "os";
 import { join } from "path";
 import { test, expect, beforeEach, afterEach, vi } from "vitest";
+import { providers as defaultProviders } from "../src/utils/providers";
 
 // In‑memory FS store
 let memfs: Record<string, string> = {};
@@ -148,3 +149,88 @@ test("loads and saves approvalMode correctly", () => {
   });
   expect(reloadedConfig.approvalMode).toBe(AutoApprovalMode.FULL_AUTO);
 });
+
+test("loads and saves providers correctly", () => {
+  // Setup custom providers configuration
+  const customProviders = {
+    openai: {
+      name: "Custom OpenAI",
+      baseURL: "https://custom-api.openai.com/v1",
+      envKey: "CUSTOM_OPENAI_API_KEY",
+    },
+    anthropic: {
+      name: "Anthropic",
+      baseURL: "https://api.anthropic.com",
+      envKey: "ANTHROPIC_API_KEY",
+    },
+  };
+
+  // Create config with providers
+  const testConfig = {
+    model: "test-model",
+    provider: "anthropic",
+    providers: customProviders,
+    instructions: "test instructions",
+    notify: false,
+  };
+
+  // Save the config
+  saveConfig(testConfig, testConfigPath, testInstructionsPath);
+
+  // Verify saved config contains providers
+  expect(memfs[testConfigPath]).toContain(`"providers"`);
+  expect(memfs[testConfigPath]).toContain(`"Custom OpenAI"`);
+  expect(memfs[testConfigPath]).toContain(`"Anthropic"`);
+  expect(memfs[testConfigPath]).toContain(`"provider": "anthropic"`);
+
+  // Load config and verify providers were loaded correctly
+  const loadedConfig = loadConfig(testConfigPath, testInstructionsPath, {
+    disableProjectDoc: true,
+  });
+
+  // Check providers were loaded correctly
+  expect(loadedConfig.provider).toBe("anthropic");
+  expect(loadedConfig.providers).toEqual({
+    ...defaultProviders,
+    ...customProviders,
+  });
+
+  // Test merging with built-in providers
+  // Create a config with only one custom provider
+  const partialProviders = {
+    customProvider: {
+      name: "Custom Provider",
+      baseURL: "https://custom-api.example.com",
+      envKey: "CUSTOM_API_KEY",
+    },
+  };
+
+  const partialConfig = {
+    model: "test-model",
+    provider: "customProvider",
+    providers: partialProviders,
+    instructions: "test instructions",
+    notify: false,
+  };
+
+  // Save the partial config
+  saveConfig(partialConfig, testConfigPath, testInstructionsPath);
+
+  // Load config and verify providers were merged with built-in providers
+  const mergedConfig = loadConfig(testConfigPath, testInstructionsPath, {
+    disableProjectDoc: true,
+  });
+
+  // Check providers is defined
+  expect(mergedConfig.providers).toBeDefined();
+
+  // Use bracket notation to access properties
+  if (mergedConfig.providers) {
+    expect(mergedConfig.providers["customProvider"]).toBeDefined();
+    expect(mergedConfig.providers["customProvider"]).toEqual(
+      partialProviders.customProvider,
+    );
+    // Built-in providers should still be there (like openai)
+    expect(mergedConfig.providers["openai"]).toBeDefined();
+  }
+});