fix(standards): reject zero-amount assets in swap and pswap notes

[Dusk1e]

Follow-up to rust-sdk#2459, where @igamigo noted these checks belong here as well.

Problem

SwapNote::new rejects a self-swap, and PswapNoteBuilder::build rejects a same-faucet pair, but neither looks at the amounts:

SwapNote::new(offered = 0, requested = <non-fungible>)      -> Ok
SwapNote::new(offered = <non-fungible>, requested = 0)      -> Ok
PswapNote::builder()...(offered = 0, requested = 100)       -> Ok
PswapNote::builder()...(offered = 100, requested = 0)       -> Ok

Both sides carry value. The note holds the offered asset, and filling it emits a payback carrying the requested one, so a zero requested asset leaves the payback empty and a zero offered asset leaves a note whose consumer pays and receives nothing.

Change

Reject a zero-amount asset on either side in both constructors, with NoteError::other alongside the existing checks.

For SWAP the assets are Asset, so the test goes through a small is_zero_fungible helper — a non-fungible asset always carries value. For PSWAP both sides are FungibleAsset and compare directly.

PswapNoteStorage::min_fill_step is left alone: it defaults to AssetAmount::ZERO on purpose, to disable the fill floor.

Test plan

cargo test -p miden-standards --lib -- builder_rejects_a_zero_asset

swap_note_builder_rejects_a_zero_asset and pswap_note_builder_rejects_a_zero_asset cover both sides of each constructor. Both fail on next — the first expect_err gets an Ok note back.

All 275 miden-standards lib tests pass, and the workspace builds.