rest: fix unreachable provably-unspendable script classification

[zloglevel]

Summary

Fix REST output script classification for provably unspendable scripts.

The existing classification contained two identical is_op_return() branches.

The second branch was unreachable, causing non-OP_RETURN provably unspendable scripts to be reported as unknown.

Changes

• Replace the unreachable duplicate is_op_return() check with is_provably_unspendable().
• Preserve the dedicated op_return classification by checking it first.
• Add regression coverage for:
  • OP_RETURN scripts
  • non-OP_RETURN provably unspendable scripts
  • unknown but potentially spendable scripts
• Cover both Bitcoin and Liquid builds.

Verification

• cargo +1.92.0 check --lib
• cargo +1.92.0 check --lib --features liquid