[oscars-integration] Thread MutationContext through buitlin standard library objects

[shruti2522]

Stacked on #5478, this PR enforces threading MutationContext through all internal builtins, removing previous temporary fallbacks.

Key Changes:

• Removed *_in methods, temporary methods like JsObject::new_in are gone. Main methods (e.g., JsObject::new) have absorbed their signatures and now require the context.
• Removed Default for core objects: StandardConstructors, IntrinsicObjects and Intrinsics can no longer use Default::default(); they must be instantiated via uninit(mc)
• Macros (eg., boa_class) were updated to dynamically fetch context.gc_collector() without violating Rust's mutable borrowing rules.
• Signatures in the CLI, examples and tests were updated. Detached tests and macros temporarily use &unsafe { boa_gc::MutationContext::global() } to minimize immediate code breakage