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/core-plugins/Cargo.toml

@@ -32,6 +32,7 @@ chrono = { workspace = true }
 dirs = { workspace = true }
 flate2 = { workspace = true }
 reqwest = { workspace = true }
+semver = { workspace = true }
 serde = { workspace = true, features = ["derive"] }
 serde_json = { workspace = true }
 tar = { workspace = true }