fix(messages): truncate warning messages only in display (#38901)

For now, add a private "_truncate" flag to nvim_echo, using a truncation
method similar to showmode().

runtime/lua/vim/_core/defaults.lua

@@ -780,7 +780,8 @@ do
       vim.v.swapchoice = 'e' -- Choose "(E)dit".
       vim.notify(
         ('W325: Ignoring swapfile from Nvim process %d'):format(info.pid),
-        vim.log.levels.WARN
+        vim.log.levels.WARN,
+        { _truncate = true }
       )
     end,
   })
@@ -991,7 +992,8 @@ do
       then
         vim.notify(
           'defaults.lua: Did not detect DSR response from terminal. This results in a slower startup time.',
-          vim.log.levels.WARN
+          vim.log.levels.WARN,
+          { _truncate = true }
         )
       end
     end

runtime/lua/vim/_core/editor.lua

@@ -547,7 +547,11 @@ end
 ---@diagnostic disable-next-line: unused-local
 function vim.notify(msg, level, opts) -- luacheck: no unused args
   local chunks = { { msg, level == vim.log.levels.WARN and 'WarningMsg' or nil } }
-  vim.api.nvim_echo(chunks, true, { err = level == vim.log.levels.ERROR })
+  vim.api.nvim_echo(
+    chunks,
+    true,
+    { err = level == vim.log.levels.ERROR, _truncate = opts and opts._truncate }
+  )
 end
 
 do
@@ -1213,9 +1217,7 @@ end
 do
   local function truncated_echo(msg)
     -- Truncate message to avoid hit-enter-prompt
-    local max_width = vim.o.columns * math.max(vim.o.cmdheight - 1, 0) + vim.v.echospace
-    local msg_truncated = string.sub(msg, 1, max_width)
-    vim.api.nvim_echo({ { msg_truncated, 'WarningMsg' } }, true, {})
+    vim.api.nvim_echo({ { msg, 'WarningMsg' } }, true, { _truncate = true })
   end
 
   local notified = false

runtime/lua/vim/_meta/api_keysets.lua

@@ -239,6 +239,7 @@ error('Cannot require a meta file')
 --- @class vim.api.keyset.echo_opts
 --- @field err? boolean
 --- @field verbose? boolean
+--- @field _truncate? boolean
 --- @field kind? string
 --- @field id? integer|string
 --- @field title? string