Add plugin install request model

[zswang-oai]

Why

The plural install tool needs a small, extension-owned wire model before it can be exposed or executed.

What changed

• add the codex-plugin-installs-extension crate
• define plural request, result, picker entry, and picker category types
• build the exact nested elicitation metadata consumed by the install picker
• keep connector completion helpers with the metadata they describe

Stack

This is PR 2 of 7.

• Base: Support plugin install entry metadata in TUI #28798
• Next: Add plugin install request schema and validation #28802

Validation

• just test -p codex-plugin-installs-extension (6 tests)