[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/app-server-protocol/src/protocol/v2/plugin.rs

@@ -744,6 +744,15 @@ pub enum PluginSource {
         ref_name: Option<String>,
         sha: Option<String>,
     },
+    #[serde(rename_all = "camelCase")]
+    #[ts(rename_all = "camelCase")]
+    Npm {
+        package: String,
+        /// Optional npm version or version range.
+        version: Option<String>,
+        /// Optional HTTPS registry URL. Authentication stays in the user's npm config.
+        registry: Option<String>,
+    },
     /// The plugin is available in the remote catalog. Download metadata is
     /// kept server-side and is not exposed through the app-server API.
     Remote,