fix(lua): use rawget() to get __call in vim.is_callable() (#29536)

Lua 5.1 uses a "raw get" to retrieve `__call` from a metatable to
determine if a table is callable. Mirror this behavior in
`vim.is_callable()`.

runtime/lua/vim/shared.lua

@@ -997,7 +997,7 @@ function vim.is_callable(f)
   if m == nil then
     return false
   end
-  return type(m.__call) == 'function'
+  return type(rawget(m, '__call')) == 'function'
 end
 
 --- Creates a table whose missing keys are provided by {createfn} (like Python's "defaultdict").

test/functional/lua/vim_spec.lua

@@ -1495,6 +1495,60 @@ describe('lua stdlib', function()
     ]])
     )
 
+    eq(
+      { false, false },
+      exec_lua([[
+      local meta = { __call = {} }
+      assert(meta.__call)
+      local function new()
+        return setmetatable({}, meta)
+      end
+      local not_callable = new()
+      return { pcall(function() not_callable() end), vim.is_callable(not_callable) }
+    ]])
+    )
+    eq(
+      { false, false },
+      exec_lua([[
+      local function new()
+        return { __call = function()end }
+      end
+      local not_callable = new()
+      assert(not_callable.__call)
+      return { pcall(function() not_callable() end), vim.is_callable(not_callable) }
+    ]])
+    )
+    eq(
+      { false, false },
+      exec_lua([[
+      local meta = setmetatable(
+        { __index = { __call = function() end } },
+        { __index = { __call = function() end } }
+      )
+      assert(meta.__call)
+      local not_callable = setmetatable({}, meta)
+      assert(not_callable.__call)
+      return { pcall(function() not_callable() end), vim.is_callable(not_callable) }
+    ]])
+    )
+    eq(
+      { false, false },
+      exec_lua([[
+      local meta = setmetatable({
+        __index = function()
+          return function() end
+        end,
+      }, {
+        __index = function()
+          return function() end
+        end,
+      })
+      assert(meta.__call)
+      local not_callable = setmetatable({}, meta)
+      assert(not_callable.__call)
+      return { pcall(function() not_callable() end), vim.is_callable(not_callable) }
+    ]])
+    )
     eq(false, exec_lua('return vim.is_callable(1)'))
     eq(false, exec_lua("return vim.is_callable('foo')"))
     eq(false, exec_lua('return vim.is_callable({})'))