[core] Allow plugin guidance to drive app installs

[adaley-openai]

Summary

• include plugin-declared app IDs, including templates, in explicit plugin invocation guidance
• allow request_plugin_install to use a concrete app ID supplied by active plugin guidance without requiring list_available_plugins_to_install first
• retain list-based discovery for unrelated installs and explicitly reject templated_apps_* IDs as install arguments

Notes

• the direct path still depends on session-loaded plugin app IDs being included in the tool-suggest candidate allowlist; that fix is tracked separately in [codex] Update remote connector suggestions #25172

Validation

• just fmt
• git diff --check
• cargo check -p codex-core
• not run: just test -p codex-core request_plugin_install (cargo-nextest is not installed locally)