fix: preserve parentheses for single-element type packs in explicit type instantiation

[JohnnyMorganz]

Fixes #1089.

Problem

f<<(number)>>(10) was incorrectly formatted to f<<number>>(10), stripping the parentheses from the single-element type pack. This changes the semantics: (number) is a type pack, while number is a plain type — using number without parentheses is a type error.

Multi-element type packs like (string, number) were unaffected because TypeInfo::Tuple with multiple elements always keeps parentheses.

Root Cause

format_type_instantiation called format_type_info with the default context (no within_generic flag). The keep_parentheses function in luau.rs uses within_generic to decide whether to strip parentheses from single-type tuples. Without this flag, (number) → number.

Fix

Apply TypeInfoContext::new().mark_within_generic() when formatting types inside <<...>>, exactly mirroring how format_type_info_generics handles regular generic type parameters like Foo<(string)>.

Test Plan

• Added tests/inputs-luau/type-instantiation-type-pack.lua with the failing case from the issue
• Red: test failed showing f<<number>>(10) (parentheses stripped)
• Green: after fix, test produces f<<(number)>>(10) (parentheses preserved)
• Full test suite passes: cargo test --features luau