feat: add GitHub Enterprise Server support

Add support for GitHub Enterprise Server instances in addition to GitHub.com:

- Add dynamic URL construction based on provider config and auth data
- Read enterpriseUrl from auth data (priority 1) or provider options (priority 2)
- Support both github-copilot and github-copilot-enterprise provider IDs
- Normalize enterprise URLs (remove protocol/trailing slashes)

The plugin integrates with core's handleGithubCopilotAuth() which prompts
users for deployment type and enterprise URL, storing it in auth data.

Priority order for enterpriseUrl:
1. Auth data (set by core during interactive authentication)
2. Provider config options
3. Default to github.com

This enables enterprise users to authenticate with their GitHub Enterprise
Server Copilot instances while maintaining excellent UX.

Author: OlaHulleberg

index.mjs

@@ -3,17 +3,57 @@
  */
 export async function CopilotAuthPlugin({ client }) {
   const CLIENT_ID = "Iv1.b507a08c87ecfe98";
-  const DEVICE_CODE_URL = "https://github.com/login/device/code";
-  const ACCESS_TOKEN_URL = "https://github.com/login/oauth/access_token";
-  const COPILOT_API_KEY_URL =
-    "https://api.github.com/copilot_internal/v2/token";
   const HEADERS = {
     "User-Agent": "GitHubCopilotChat/0.35.0",
     "Editor-Version": "vscode/1.99.3",
     "Editor-Plugin-Version": "copilot-chat/0.35.0",
     "Copilot-Integration-Id": "vscode-chat",
   };
 
+  /**
+   * Normalizes a domain by removing protocol and trailing slashes
+   */
+  function normalizeDomain(url) {
+    return url
+      .replace(/^https?:\/\//, "")
+      .replace(/\/$/, "");
+  }
+
+  /**
+   * Gets the base URL from auth data, provider config, or defaults to github.com
+   * Priority: auth data > config > github.com
+   */
+  async function getBaseUrl(providerId, authInfo) {
+    try {
+      // First check auth data (set by core during authentication)
+      if (authInfo && authInfo.enterpriseUrl) {
+        return normalizeDomain(authInfo.enterpriseUrl);
+      }
+
+      // Then check config
+      const config = await client.config.get();
+      const providerConfig = config?.provider?.[providerId];
+      const configUrl = providerConfig?.options?.enterpriseUrl;
+
+      return configUrl ? normalizeDomain(configUrl) : "github.com";
+    } catch {
+      return "github.com";
+    }
+  }
+
+  /**
+   * Constructs URLs based on the base URL (github.com or enterprise)
+   */
+  async function getUrls(providerId, authInfo) {
+    const baseUrl = await getBaseUrl(providerId, authInfo);
+
+    return {
+      DEVICE_CODE_URL: `https://${baseUrl}/login/device/code`,
+      ACCESS_TOKEN_URL: `https://${baseUrl}/login/oauth/access_token`,
+      COPILOT_API_KEY_URL: `https://api.${baseUrl}/copilot_internal/v2/token`,
+    };
+  }
+
   return {
     auth: {
       provider: "github-copilot",
@@ -36,7 +76,8 @@ export async function CopilotAuthPlugin({ client }) {
             const info = await getAuth();
             if (info.type !== "oauth") return {};
             if (!info.access || info.expires < Date.now()) {
-              const response = await fetch(COPILOT_API_KEY_URL, {
+              const urls = await getUrls(provider.id, info);
+              const response = await fetch(urls.COPILOT_API_KEY_URL, {
                 headers: {
                   Accept: "application/json",
                   Authorization: `Bearer ${info.refresh}`,
@@ -102,7 +143,9 @@ export async function CopilotAuthPlugin({ client }) {
           label: "Login with GitHub",
           type: "oauth",
           authorize: async () => {
-            const deviceResponse = await fetch(DEVICE_CODE_URL, {
+            // During authorize, read from config only (no auth data exists yet)
+            const urls = await getUrls("github-copilot", null);
+            const deviceResponse = await fetch(urls.DEVICE_CODE_URL, {
               method: "POST",
               headers: {
                 Accept: "application/json",
@@ -121,7 +164,7 @@ export async function CopilotAuthPlugin({ client }) {
               method: "auto",
               callback: async () => {
                 while (true) {
-                  const response = await fetch(ACCESS_TOKEN_URL, {
+                  const response = await fetch(urls.ACCESS_TOKEN_URL, {
                     method: "POST",
                     headers: {
                       Accept: "application/json",