[Feature Request] Allow mark any as @enum

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### Issue Description

```lua
---@enum Type
local Type = {
A = 1,
B = 2,
-- ...
}

---@return {[string]: integer}
function Generate_enum()
return {}
end

---@enum (partial) Type
local Type2 = {}

for k, v in pairs(Generate_enum()) do
Type2[k] = v
end

---@enum (partial) Type
local Type3 = Generate_enum()

```



The point is to achieve the same behavior for `Type3` as for `Type2`.

Start by reproducing the issue's Lua examples and inspect the language server's existing @enum handling and type inference for assignments. Compare Type2 and Type3, then add coverage so a partial enum assigned directly from Generate_enum() receives the same behavior as the incremental assignment; done means their diagnostics and type information match.

Written by the indexing model from the issue text.