chore(copilot): use GH_HOST for Copilot endpoints and token lookup

Update Copilot HTTP requests and token lookup to respect the `GH_HOST` env var, allowing use with
GitHub Enterprise instances.

Some networks block the `github.com` endpoints, so they won't work at all. Provide a way for the
user to hit different endpoint other than `github.com` (e.g. mycorp.ghe.com) without patching the
source code.

The `GH_HOST` environment variable is the same one used to override this for `gh` cli:
https://cli.github.com/manual/gh_help_environment

Author: rmuir

lua/codecompanion/adapters/http/copilot/init.lua

@@ -33,8 +33,11 @@ end
 ---@return table
 local function handlers(adapter)
   local model_opts = resolve_model_opts(adapter)
+  local current_url = adapter.url or "https://api.githubcopilot.com/chat/completions"
+  local base_url = current_url:gsub("/chat/completions$", ""):gsub("/responses$", "")
+
   if model_opts.endpoint == "responses" then
-    adapter.url = "https://api.githubcopilot.com/responses"
+    adapter.url = base_url .. "/responses"
 
     local responses = require("codecompanion.adapters.http.openai_responses")
 
@@ -73,7 +76,7 @@ local function handlers(adapter)
     return responses.handlers
   end
 
-  adapter.url = "https://api.githubcopilot.com/chat/completions"
+  adapter.url = base_url .. "/chat/completions"
   return require("codecompanion.adapters.http.openai").handlers
 end
 

lua/codecompanion/adapters/http/copilot/stats.lua

@@ -27,8 +27,17 @@ local function get_statistics()
 
   local oauth_token = token.fetch({ force = true }).oauth_token
 
+  local host = vim.env.GH_HOST or "github.com"
+  local endpoint
+  if host == "github.com" then
+    endpoint = "https://api.github.com/copilot_internal/v2/token"
+  else
+    -- GitHub Enterprise usually puts the API under /api/v3
+    endpoint = string.format("https://%s/api/v3/copilot_internal/v2/token", host)
+  end
+
   local ok, response = pcall(function()
-    return Curl.get("https://api.github.com/copilot_internal/user", {
+    return Curl.get(endpoint, {
       sync = true,
       headers = {
         Authorization = "Bearer " .. oauth_token,

lua/codecompanion/adapters/http/copilot/token.lua

@@ -104,7 +104,7 @@ local function get_oauth_token()
 
       userdata = vim.json.decode(userdata)
       for key, value in pairs(userdata) do
-        if string.find(key, "github.com") then
+        if string.find(key, vim.env.GH_HOST or "github.com") then
           return value.oauth_token
         end
       end
@@ -136,8 +136,17 @@ local function get_copilot_token()
   _token_fetch_in_progress = true
   log:trace("Authorizing GitHub Copilot token")
 
+  local host = vim.env.GH_HOST or "github.com"
+  local endpoint
+  if host == "github.com" then
+    endpoint = "https://api.github.com/copilot_internal/v2/token"
+  else
+    -- GitHub Enterprise usually puts the API under /api/v3
+    endpoint = string.format("https://%s/api/v3/copilot_internal/v2/token", host)
+  end
+
   local ok, request = pcall(function()
-    return Curl.get("https://api.github.com/copilot_internal/v2/token", {
+    return Curl.get(endpoint, {
       headers = {
         Authorization = "Bearer " .. (M._oauth_token or ""),
         Accept = "application/json",