jsc: replace MarkedArrayBuffer::from_bytes with an owning constructor - #31986

The fixture's committed `Cargo.lock` records the dependency lists of ~80 `bun_*` path crates (bun_jsc transitively pulls in bun_bundler, bun_install, bun_http, etc.), so with `--locked` any PR that adds/removes/bumps a dep in any of those crates' Cargo.toml will fail this test with "the lock file needs to be updated but --locked was passed" — exit 101, no `E0599`/`from_bytes` in the output, so the assertion at lines 61-65 reports only `{rejectsFromBytes: false}` with no hint that the fix is `car

The SAFETY comment claims "`owns_buffer` is only set by `from_owned_bytes`", but `owns_buffer` is `pub` and `src/runtime/api/bun/subprocess/Readable.rs:313-317` constructs a `MarkedArrayBuffer { …, owns_buffer: true, … }` struct literal directly. The underlying invariant still holds there (it goes through `ArrayBuffer::from_owned_bytes`), so this is not a soundness issue — but since this PR's stated goal is to route every caller through the typed constructor, it'd be cleanest to migrate that sit