fix(plugins): keep version upgrades additive

[iceweasel-oai]

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

[chatgpt-codex-connector]

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