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

test/functional/lua/net_spec.lua

@@ -0,0 +1,60 @@
+local n = require('test.functional.testnvim')()
+local t = require('test.testutil')
+local skip_integ = os.getenv('NVIM_TEST_INTEG') ~= '1'
+
+local exec_lua = n.exec_lua
+
+local function assert_404_error(err)
+  assert(
+    err:lower():find('404') or err:find('22'),
+    'Expected HTTP 404 or exit code 22, got: ' .. tostring(err)
+  )
+end
+
+describe('vim.net.request', function()
+  before_each(function()
+    n:clear()
+  end)
+
+  it('fetches a URL into memory (async success)', function()
+    t.skip(skip_integ, 'NVIM_TEST_INTEG not set: skipping network integration test')
+    local content = exec_lua([[
+      local done = false
+      local result
+      local M = require('vim.net')
+
+      M.request("https://httpbingo.org/anything", { retry = 3 }, function(err, body)
+        assert(not err, err)
+        result = body.body
+        done = true
+      end)
+
+      vim.wait(2000, function() return done end)
+      return result
+    ]])
+
+    assert(
+      content and content:find('"url"%s*:%s*"https://httpbingo.org/anything"'),
+      'Expected response body to contain the correct URL'
+    )
+  end)
+
+  it('calls on_response with error on 404 (async failure)', function()
+    t.skip(skip_integ, 'NVIM_TEST_INTEG not set: skipping network integration test')
+    local err = exec_lua([[
+      local done = false
+      local result
+      local M = require('vim.net')
+
+      M.request("https://httpbingo.org/status/404", {}, function(e, _)
+        result = e
+        done = true
+      end)
+
+      vim.wait(2000, function() return done end)
+      return result
+    ]])
+
+    assert_404_error(err)
+  end)
+end)