Fix Lua language server startup when offline

[cmdr-chara]

Summary

I used AI assistance to develop this fix, reviewed the changes, and tested it offline on Linux.

• Resolve an explicit or PATH-provided server without eagerly invoking managed installation.
• Fall back to the newest complete stable LuaLS installation on disk when the update request fails after restarting Zed.
• Stage downloads before promotion so failed extraction does not expose an incomplete cached installation.

Addresses the Lua-specific case of zed-industries/zed#9789; does not resolve the broader issue. Cold startup still waits for the update request to fail before falling back.

Verification

• 15 standalone Rust tests pass, compiling the actual binary-resolution module with warnings denied.
• Rust formatting and whitespace checks pass.
• I installed the development extension in Zed on Linux and confirmed that completions and documentation worked after restarting offline, as shown in the screenshot below.
• Full cross-platform validation remains outstanding; Windows filenames were tested on Linux, not on Windows. No Zed rebuild was performed.

Screenshot

Tested on Linux: fully quit Zed, disconnected from the internet, and reopened the Lua file. Completions and documentation worked after the update request timed out.

Release Notes:

• Fixed Lua language server startup when offline and a previously downloaded server is available.