feat(nvim): collapse neotest diagnostic virtual text to one line

[fredrikaverpil]

Why?

• neotest renders diagnostic messages as virtual text, but testify failures are full of tabs and newlines, so the virtual text is barely readable.
• Supersedes feat(nvim): neotest improved diagnostics #151, which made the same improvement against the old lazy.nvim config (lua/plugins/neotest.lua) that no longer exists after the migration to native vim.pack.

What?

• Register a vim.diagnostic format function on the neotest namespace that strips newlines/tabs and squeezes runs of whitespace, in the current plugin/neotest.lua.

local neotest_ns = vim.api.nvim_create_namespace("neotest")
vim.diagnostic.config({
  virtual_text = {
    format = function(diagnostic)
      return diagnostic.message:gsub("\n", " "):gsub("\t", " "):gsub("%s+", " "):gsub("^%s+", "")
    end,
  },
}, neotest_ns)

Notes

• Verified live in headless Neovim: a testify-style message collapses from \n\tError Trace:\tfoo_test.go:42\n\tError:\t\tNot equal:\n... to Error Trace: foo_test.go:42 Error: Not equal: expected: 3 actual : 4 Test: TestAdd — no newlines, tabs, double-spaces, or leading whitespace.
• Closes feat(nvim): neotest improved diagnostics #151 in favor of this PR.