[codex] Canonicalize shared workspace plugin IDs

[xl-openai]

Summary

• Canonicalize private and unlisted workspace shared plugin IDs to workspace-shared-with-me.
• Keep plugin/list private/unlisted shared-with-me buckets as UI grouping only.
• Update share read/list/checkout and cache cleanup coverage for the canonical namespace.

Tests

• cargo test -p codex-app-server --test all plugin_list_fetches_shared_with_me_kind
• cargo test -p codex-app-server --test all plugin_read_returns_share_context_for_shared_remote_plugin
• cargo test -p codex-app-server --test all suite::v2::plugin_share
• cargo test -p codex-core-plugins list_remote_plugin_shares_fetches_created_workspace_plugins
• cargo test -p codex-core-plugins stale_remote_plugin_cleanup_removes_old_shared_with_me_cache_and_keeps_canonical_cache
• git diff --check