Add plugin install extension executor

[zswang-oai]

Why

The extension needs a small executor boundary so core can provide host behavior without owning the tool schema or argument validation.

What changed

• add RequestPluginInstallsBackend and its request/future contract
• add the extension ToolExecutor
• parse arguments, validate them, delegate to the backend, and serialize the model-visible result

Stack

This is PR 4 of 7.

• Base: Add plugin install request schema and validation #28802
• Next: Add plugin install extension backend #28818

Validation

• just test -p codex-plugin-installs-extension (15 tests)