fix(type): preserve unknown unions and narrow pcall(any)

[lewis6991]

Preserve unknown as a real union member in type unions and doc-type
parsing instead of collapsing unknown|T to T.

Update call sites that previously used Unknown as an empty union
accumulator to use Never or explicit first-value handling, and avoid
merging missing closure param annotations as unknown.

Keep the pcall(any) flow fixes so the payload stays unknown|string
outside the branch, narrows to unknown on success, and narrows to string
on failure.

---

This fixes local ok, result = pcall(x) where x:any so the success branch
sees unknown and the failure branch sees string.

---@type any
local x

local ok, result = pcall(x)
if ok then
    success = result -- should be unknown
else
    failure = result -- should be string
end

[gemini-code-assist]

Code Review

This pull request improves the handling of never types within unions and simplifies the processing of MultiLineUnion types across the codebase. Specifically, LuaType::from_vec now correctly preserves LuaType::Never when a union would otherwise be empty, and several semantic analysis functions were refactored to recursively handle the results of MultiLineUnion::to_union. Additionally, new test cases were added to verify type preservation for multiline aliases and type narrowing with pcall. I have no feedback to provide.