fix(rust): drop incompatible root_dir under native_lsp_config

[rami3l]

📑 Description

With astrolsp native_lsp_config enabled, lsp_opts("rust_analyzer") returns the full vim.lsp.config table including nvim-lspconfig's root_dir(bufnr, on_dir) function. rustaceanvim calls its root_dir with (file_name, default_fn) instead, causing on every rust file open:

Error executing lua callback: .../lsp/rust_analyzer.lua:89: Invalid 'buffer': Expected Lua number
stack traceback:
        [C]: in function 'nvim_buf_get_name'
        .../nvim-lspconfig/lsp/rust_analyzer.lua:89: in function 'root_dir'
        .../rustaceanvim/lua/rustaceanvim/cargo.lua:127: in function 'get_config_root_dir'
        .../rustaceanvim/lua/rustaceanvim/lsp/init.lua:171: in function 'start'

This drops root_dir from the forwarded opts so rustaceanvim falls back to its own cargo-aware root detection.

📖 Additional Information

Replaces #1747 since some details have been changed after upgrading to AstroNvim v6.

Original commit from @Mic92.

cc @Uzaaft

[github-actions]

Review Checklist

Does this PR follow the [Contribution Guidelines](development guidelines)? Following is a partial checklist:

Proper conventional commit scoping:

• If you are adding a new plugin, the scope would be the name of the category it is being added into. ex. feat(utility): added noice.nvim plugin

• If you are modifying a pre-existing plugin or pack, the scope would be the name of the plugin folder. ex. fix(noice-nvim): fix LSP handler error

• Pull request title has the appropriate conventional commit type and scope where the scope is the name of the pre-existing directory in the project as described above

• README is properly formatted and uses fenced in links with <url> unless they are inside a [title](url)

• Entry returns a single plugin spec with the new plugin as the only top level spec (not applicable for recipes or packs).

• Proper usage of opts table rather than setting things up with the config function.

• Proper usage of specs table for all specs that are not dependencies of a given plugin (not applicable for recipes or packs).