[codex] expose remote plugin share URL (#27890)

## Summary

- expose the remote plugin detail endpoint's `share_url` as nullable
`PluginDetail.shareUrl`
- preserve existing `PluginSummary.shareContext` behavior for local and
workspace sharing flows
- regenerate the app-server TypeScript and JSON schema fixtures

## Why

The remote plugin detail response already includes a canonical
`share_url`, but that value was not surfaced by `plugin/read` for global
plugins. Global plugins intentionally have no `shareContext`, so using
that model for the URL would change the semantics consumed by the
existing share modal.

## User impact

Codex clients can use `PluginDetail.shareUrl` for a remote plugin's
copy-link action, including when the plugin is disabled by an
administrator, without changing existing share-modal or ownership
behavior.

## Validation

- `cargo test -p codex-app-server
plugin_read_includes_share_url_for_admin_disabled_remote_plugin`
- `cargo test -p codex-app-server-protocol
typescript_schema_fixtures_match_generated`
- `cargo test -p codex-app-server-protocol
json_schema_fixtures_match_generated`
- `cargo fmt --all`

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

@@ -638,6 +638,7 @@ pub struct PluginDetail {
     pub marketplace_name: String,
     pub marketplace_path: Option<AbsolutePathBuf>,
     pub summary: PluginSummary,
+    pub share_url: Option<String>,
     pub description: Option<String>,
     pub skills: Vec<SkillSummary>,
     pub hooks: Vec<PluginHookSummary>,