[codex] Support npm marketplace plugin sources (#29375)

## Why

Marketplace source deserialization treated `{"source":"npm", ...}` as
unsupported. The loader logged and skipped the entry, so npm-backed
plugins never appeared in `plugin list --available` and `plugin add`
returned "plugin not found".

Codex plugins are installed from a plugin root, not from an npm
dependency tree. For npm-backed marketplace entries, Codex should fetch
the published package contents without running package scripts or
installing unrelated dependencies.

## What changed

- Add `npm` marketplace plugin sources with `package`, optional semver
`version` or version range, and optional HTTPS `registry`.
- Reject unsafe npm source fields before materialization, including
invalid package names, non-semver version selectors, plaintext or
credential-bearing registry URLs, and registry query/fragment data.
- Materialize npm plugins with `npm pack --ignore-scripts`, then unpack
the resulting tarball through the existing hardened plugin bundle
extractor.
- Enforce npm archive and extracted-size limits, require the standard
npm `package/` archive root, and verify the extracted `package.json`
name matches the requested package before installing.
- Keep plugin listings, install-source descriptions, CLI JSON/human
output, app-server v2 `PluginSource`, TUI source summaries, regenerated
schema fixtures, and app-server documentation in sync.

## Impact

Marketplaces can distribute Codex plugins from public or configured
private HTTPS npm registries using the same install flow as existing
materialized plugin sources. `npm` must be available on `PATH` when an
npm-backed plugin is installed.

Fixes #27831

## Validation

- `just write-app-server-schema`
- `just test -p codex-core-plugins -p codex-app-server-protocol -p
codex-app-server -p codex-cli`
  - npm/schema/core-plugin coverage passed in the run.
- The full focused command finished with `1739 passed`, `11 failed`, and
`6 timed out`; the failures were unrelated local app-server environment
failures from `sandbox-exec: sandbox_apply: Operation not permitted`
plus one missing `test_stdio_server` helper binary.
- Installed an npm-published Codex plugin package through a throwaway
local marketplace and throwaway `CODEX_HOME` to exercise the real npm
materialization path end to end.

codex-rs/cli/src/plugin_cmd.rs

@@ -285,6 +285,20 @@ pub async fn run_plugin_list(
                         }
                         parts.join(", ")
                     }
+                    codex_core_plugins::marketplace::MarketplacePluginSource::Npm {
+                        package,
+                        version,
+                        registry,
+                    } => {
+                        let mut parts = vec![package.clone()];
+                        if let Some(version) = version {
+                            parts.push(format!("version `{version}`"));
+                        }
+                        if let Some(registry) = registry {
+                            parts.push(format!("registry `{registry}`"));
+                        }
+                        parts.join(", ")
+                    }
                 };
                 plugin_width = plugin_width.max(plugin.id.len());
                 status_width = status_width.max(state.len());
@@ -412,6 +426,13 @@ enum JsonPluginSource {
         #[serde(skip_serializing_if = "Option::is_none")]
         sha: Option<String>,
     },
+    Npm {
+        package: String,
+        #[serde(skip_serializing_if = "Option::is_none")]
+        version: Option<String>,
+        #[serde(skip_serializing_if = "Option::is_none")]
+        registry: Option<String>,
+    },
 }
 
 impl JsonPluginSource {
@@ -437,6 +458,15 @@ impl JsonPluginSource {
                 ref_name,
                 sha,
             } => Self::Git { url, ref_name, sha },
+            MarketplacePluginSource::Npm {
+                package,
+                version,
+                registry,
+            } => Self::Npm {
+                package,
+                version,
+                registry,
+            },
         }
     }
 }