feat(net): vim.net.request(), :edit [url] #34140

Problem:
Nvim depends on netrw to download/request URL contents.

Solution:
- Add `vim.net.request()` as a thin curl wrapper:
  - Basic GET with --silent, --show-error, --fail, --location, --retry
  - Optional `opts.outpath` to save to a file
  - Operates asynchronously. Pass an `on_response` handler to get the result.
- Add integ tests (requires NVIM_TEST_INTEG to be set) to test success
  and 404 failure.
- Health check for missing `curl`.
- Handle `:edit https://…` using `vim.net.request()`.

API Usage:
1. Asynchronous request:

    vim.net.request('https://httpbingo.org/get', { retry = 2 }, function(err, response)
      if err then
        print('Fetch failed:', err)
      else
        print('Got body of length:', #response.body)
      end
    end)

2. Download to file:

    vim.net.request('https://httpbingo.org/get', { outpath = 'out_async.txt' }, function(err)
      if err then print('Error:', err) end
    end)

3. Remote :edit integration (in runtime/plugin/net.lua) fetches into buffer:

    :edit https://httpbingo.org/get

runtime/lua/vim/net.lua

@@ -0,0 +1,63 @@
+local M = {}
+
+--- Makes an HTTP GET request to the given URL (asynchronous).
+---
+--- This function operates in one mode:
+---   - Asynchronous (non-blocking): Returns immediately and passes the response object to the
+---   provided `on_response` handler on completetion.
+---
+--- @param url string The URL for the request.
+--- @param opts? table Optional parameters:
+---   - `verbose` (boolean|nil): Enables verbose output.
+---   - `retry`   (integer|nil): Number of retries on transient failures (default: 3).
+---   - `outpath`  (string|nil): File path to save the response body to. If set, the `body` value in the Response Object will be `true` instead of the response body.
+--- @param on_response fun(err?: string, response?: { body: string|boolean }) Callback invoked on request
+--- completetion. The `body` field in the response object contains the raw response data (text or binary).
+--- Called with (err, nil) on failure, or (nil, { body = string|boolean }) on success.
+function M.request(url, opts, on_response)
+  vim.validate({
+    url = { url, 'string' },
+    opts = { opts, 'table', true },
+    on_response = { on_response, 'function' },
+  })
+
+  opts = opts or {}
+  local retry = opts.retry or 3
+
+  -- Build curl command
+  local args = { 'curl' }
+  if opts.verbose then
+    table.insert(args, '--verbose')
+  else
+    vim.list_extend(args, { '--silent', '--show-error', '--fail' })
+  end
+  vim.list_extend(args, { '--location', '--retry', tostring(retry) })
+
+  if opts.outpath then
+    vim.list_extend(args, { '--output', opts.outpath })
+  end
+
+  table.insert(args, url)
+
+  local function on_exit(res)
+    local err_msg = nil
+    local response = nil
+
+    if res.code ~= 0 then
+      err_msg = (res.stderr ~= '' and res.stderr)
+        or string.format('Request failed with exit code %d', res.code)
+    else
+      response = {
+        body = opts.outpath and true or res.stdout,
+      }
+    end
+
+    if on_response then
+      on_response(err_msg, response)
+    end
+  end
+
+  vim.system(args, {}, on_exit)
+end
+
+return M