chore: use Cow in scalars/HexString

[grandima]

Linked Issues/PRs

close https://github.com/FuelLabs/fuel-core/issues/2657

Description

Checklist

• [ ] Breaking changes are clearly marked as such in the PR description and changelog
• [x] New behavior is reflected in tests
• [ ] The specification matches the implemented behavior (link update PR if changes are needed)

Before requesting review

• [x] I have reviewed the code myself
• [ ] I have created follow-up issues caused by this PR and linked them here

After merging, notify other teams

[Add or remove entries as needed]

• [ ] Rust SDK
• [ ] Sway compiler
• [ ] Platform documentation (for out-of-organization contributors, the person merging the PR will do this)
• [ ] Someone else?

---

[!NOTE] Replaces owned HexString with Cow-backed HexString<'_> and updates GraphQL types/methods to borrow bytes where possible, reducing clones and adding required lifetimes.

• Scalars
  • Replace HexString with HexString<'a> backed by Cow<[u8]>.
  • Add conversions: From<Vec<u8>>, From<&[u8]>, From<HexString> for Vec<u8>; update Display, ScalarType, CursorType, FromStr, and Nonce conversions to support borrowing.
• GraphQL Schema Updates
  • Change return/input types from HexString to HexString<'_> across schema: blob, contract, da_compressed, message, storage, tx (queries, mutations, subscriptions, types, receipts), and upgrades.
  • Introduce lifetimes in types and unions where needed (e.g., Input<'a>, InputCoin<'a>, InputMessage<'a>, UploadedBytecode<'a>, subscriptions).
  • Construct HexString via borrowing (HexString::from(&[..]) / .into()) and remove unnecessary clones in mappers (e.g., witnesses, scripts, predicate/data, storage slots, block/tx streams).
  • Adjust pagination and connection cursors to use HexString<'_> where relevant.

^{Written by Cursor Bugbot for commit ae7848d887620ce4be7ddc21a087ae86ccff2b12. This will update automatically on new commits. Configure here.}