feat: add .npmrc support for private npm registries (#8039)

* feat: add .npmrc support for private npm registries

Add a new `npmrc` instance setting that accepts full .npmrc file content
for configuring private npm registries. Works with bun (native .npmrc
support since 1.1.18), deno (native .npmrc support in 2.x), and the npm
proxy (parses default registry + auth token from .npmrc).

Legacy `npm_config_registry` and `bunfig_install_scopes` fields are now
hidden when empty, so new users only see the .npmrc field. Also fixes a
pre-existing race condition where gen_bunfig was called after
start_child_process.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* all

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/backend-test.yml

@@ -165,6 +165,12 @@ jobs:
           fi
 
           echo "NPM_TOKEN=${NPM_TOKEN}" >> $GITHUB_ENV
+          {
+            echo "TEST_NPMRC<<NPMRC_EOF"
+            echo "@windmill-test:registry=http://localhost:4873/"
+            echo "//localhost:4873/:_authToken=${NPM_TOKEN}"
+            echo "NPMRC_EOF"
+          } >> $GITHUB_ENV
           echo "Got NPM token successfully: ${NPM_TOKEN:0:10}..."
 
           # Configure npm globally with the auth token