[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/src/request_processors/plugins.rs

@@ -101,6 +101,15 @@ fn marketplace_plugin_source_to_info(source: MarketplacePluginSource) -> PluginS
             ref_name,
             sha,
         },
+        MarketplacePluginSource::Npm {
+            package,
+            version,
+            registry,
+        } => PluginSource::Npm {
+            package,
+            version,
+            registry,
+        },
     }
 }
 
@@ -134,7 +143,7 @@ fn share_context_for_source(
                 creator_name: None,
                 share_principals: None,
             }),
-        MarketplacePluginSource::Git { .. } => None,
+        MarketplacePluginSource::Git { .. } | MarketplacePluginSource::Npm { .. } => None,
     }
 }