Problem: Repeated default fields and field-by-field assertions obscure
the values that each functional test is meant to check.

Solution: Add eq_partial() to check selected record fields while
preserving list length and order. Use local helpers to fill fixed
expected defaults where tests need full equality. Keep explicit absence
checks and test-specific values at the call sites.

AI-assisted

Problem:
Cannot assign Lua functions/closures to "func" ('completefunc',
'tagfun', …) or "expr" ('foldexpr', 'indentexpr', …) options.

Solution:
- Store "func"/"expr" options as `Callback` instead of string.
- Delete oceans of copy-pasted code.
- BREAKING: LuaRef returned via RPC/Vimscript is now represented as
  `"<Lua N: file:line>"` (like what `:map` shows) instead of `nil`.
- Note: `man.vim` still uses `v:lua` string, bc it's a vimscript ftplugin.

Helped-by: Lewis Russell <lewis6991@gmail.com>

Problem:
The magic globals `it`, `describe`, etc., are more trouble than they are
worth.

- Hooking into `after_each` requires `getfenv()` hacks.
- They confuse luals/emmylua, because the top-level `.luarc.json` isn't
  merged with `test/.luarc.json` (apparently a luals limitation?)
- They totally defeat discoverability because the user just has to
  "know" about the various magic symbols.

So they harm DX, which means they serve no purpose at all.

Solution:
- Expose the test API from `testutil`, so tests can call `t.it()`,
  `t.describe()`, etc., in the conventional way.
- Drop `getfenv()` hacks.
- Drop the `setfenv()` injection in `load_chunk`.
- Drop `test/_meta.lua`.

problem:
this crashes neovim:

ondska = setmetatable({}, {__tostring = error})
error(ondska)

solution:

make it not crash neovim

Co-authored-by: michael-grunder <michael.grunder@gmail.com>
Co-authored-by: Olivia Kinnear <git@superatomic.dev>
Co-authored-by: Felipe Matarazzo <felipemps@protonmail.com>

Problem:
Can't get a command's description from nvim_get_commands when
cmd is string.

Solution:
Returns "desc" field in nvim_get_commands.
`definition` is now empty when cmd is function type.

Problem:
After 55ceb31,  z= and tselect don't work if `vim.ui.select` is an async
provider (especially terminal buffers).

Solution:
Drop the `vim.wait()` approach, use an async approach.

fix #39506

Problem:
Invalid `nvim_create_user_command` calls can leak the
`preview` callback reference after Neovim has taken ownership of it.

1. build with {a,l}san
2. run:
    ```sh
    <path/to/nvim> --headless -u NONE --clean +'lua
    for i = 1, 100 do
      pcall(vim.api.nvim_create_user_command,
        "some very epic stuff" .. i,
        {}, -- NOTE: this is INVALID (not a function or string)
        { preview = function() end })
    end
    vim.cmd("qa!")
    ' +qa
    ```
3. see:
    ```
    100 lua references were leaked!
    ```

Solution:
Clear `preview_luaref` in `err:`.

fix https://github.com/neovim/neovim.github.io/issues/419

Co-authored-by: Rob Pilling <robpilling@gmail.com>

Problem:
nvim_get_commands does not return callbacks defined for
"preview", "complete", or the command itself.

Solution:
- Return Lua function as "callback" field in a Lua context.
- Return "preview" function in a Lua context.
- BREAKING: Return "complete" as a function instead of a boolean.