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

[EddieHouston]

Thank for your submission. The duplicated is_op_return() branch is clearly unreachable, and this restores the original REST distinction between op_return and other provably-unspendable scripts. (changed with e497577)

Could we avoid introducing a new call to the deprecated bitcoin::Script::is_provably_unspendable() and suppressing it with #[allow(deprecated)]?

The same compatibility issue was handled in mempool/electrs#138 (mempool/electrs#138), specifically in commit f8302d7c (mempool/electrs@f8302d7). It defines a small feature-specific IsProvablyUnspendable helper: the Bitcoin implementation preserves the existing ReturnOp | IllegalOp behavior, while the Liquid implementation delegates to elements::Script::is_provably_unspendable().

Using the same approach here would preserve the intended API behavior without depending on a method that rust-bitcoin plans to remove.

It would also be good to run the new unit test under both the default and liquid configurations rather than only cargo check.