Remove singular plugin install tool

[zswang-oai]

Why

After core switches to the extension, the singular request_plugin_install implementation is dead code and should not remain as a second API surface.

What changed

• remove the old core handler, spec, and handler tests
• remove the singular codex-tools request model and tests
• remove the singular tool-name export

This is a mechanical cleanup layer; runtime behavior changed in the previous PR.

Stack

This is PR 7 of 7.

• Base: Use plural plugin install extension in core #28819