fix(plugins): keep version upgrades additive (#23356)

## Why

Windows can reject plugin cache upgrades when a running MCP server still
has its working directory inside the currently active plugin version.
The existing cache refresh path replaces
`plugins/cache/<marketplace>/<plugin>` as a whole, so a live handle
under the old version can make an otherwise ordinary version bump fail.

This PR keeps the existing plugin-selection model intact while making
version bumps less disruptive.

## What changed

- When installing a new version beside an existing plugin cache root,
move only the staged version directory into place instead of replacing
the whole plugin root.
- Best-effort prune older sibling version directories after the new
version is activated.
- Preserve the existing whole-root replacement path for first installs
and same-version refreshes.
- Add regression coverage for upgrading from `1.0.0` to `2.0.0` without
replacing the plugin root.

## Verification

- `cargo test -p codex-core-plugins install_with_new_version`
- `cargo fmt --package codex-core-plugins --check`

codex-rs/Cargo.lock

@@ -2636,6 +2636,7 @@ dependencies = [
  "libc",
  "pretty_assertions",
  "reqwest",
+ "semver",
  "serde",
  "serde_json",
  "tar",