coins/refactor: enforce GetCoin() returns only unspent coins
3bc868019932
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Description

coins/refactor: enforce GetCoin() returns only unspent coins

Summary:

txdb: assert CCoinsViewDB::GetCoin only returns unspent coins

The chainstate UTXO database only stores unspent outputs; spent entries are removed.

Assert after reading a Coin so corruption or misuse cannot propagate a spent coin through the GetCoin() interface.

test: do not return spent coins from CCoinsViewTest::GetCoin

Production GetCoin() implementations only return unspent coins.

Update the CCoinsView test backend to match that contract, so tests stop exercising cache states that cannot occur with CCoinsViewCache or CCoinsViewDB.

fuzz: keep coinscache_sim backend free of spent coins

CoinsViewBottom roughly simulates a memory-backed CCoinsViewDB, which never stores spent coins.

Stop returning spent coins from GetCoin(), erase spent entries in BatchWrite(), and tighten comparisons to expect std::nullopt when the simulator has no coin.

coins: assume GetCoin only returns unspent coins

CCoinsViewCache::FetchCoin() had special handling for a spent Coin returned by the parent view.
Production parents (CCoinsViewCache and CCoinsViewDB) do not return spent coins, so this path is unreachable.

Replace it with an Assume(!coin.IsSpent()), drop outdated documentation about spent+FRESH cache entries, and simplify SanityCheck() to assert the remaining possible state invariants.
This is safe because it does not change behavior for valid backends and will fail fast if the GetCoin() contract is violated.