honor tokens-to-return on /address token balances

[pragmaxim]

Problem

Before this PR, Blockbook parsed tokens=nonzero|used|derived for address requests, but the EVM address path did not actually honor that filter.

EVM address token filtering is inconsistent with xpub filtering :

• getAccountInfo and /api/v2/address parses tokens=nonzero|used|derived but the EVM address path ignored those modes during token assembly.
• Zero-balance ERC20s and empty ERC721/ERC1155 contracts could remain in address responses even when callers wanted current holdings only.

Solution

Apply token-mode filtering directly in the EVM address token assembly path :

• Add explicit rules for ERC20, ERC721, and ERC1155 so tokens=nonzero is honored on the EVM address path.
• Preserve the legacy omission of metadata-only ERC721/ERC1155 contracts for tokens=used and tokens=derived to stay compatible with current websocket consumers.

Behavior after change

ERC20

• tokens=nonzero: now works properly. Zero-balance ERC20s are excluded when holdings are available.
• tokens=used / tokens=derived: zero-balance ERC20s are still included, to preserve legacy behavior.
• Net effect: the real functional fix for ERC20 is that nonzero finally means nonzero.

ERC721 / ERC1155

• tokens=nonzero: now works properly.
• ERC721 is included only if it has ids.
• ERC1155 is included only if it has multiTokenValues with holdings.
• tokens=used / tokens=derived: empty NFT-style contracts are still omitted, preserving previous address-path behavior.
• Net effect: nonzero becomes correct, but compatibility is kept for websocket/default consumers by not surfacing metadata-only empty NFT contracts.

One caveat

• For details=tokens, current holdings are not fetched, so tokens=nonzero behaves like used. That is documented in docs/api.md (line 479).

Practical summary

• ERC20: stricter and more correct for nonzero.
• ERC721/ERC1155: stricter for nonzero, but intentionally compatibility-preserving for used / derived.
• contract= history behavior remains intact even if a token is filtered out of the tokens list.

Test Commands

• go test -tags unittest ./api -run 'TestHasEthereumTokenHoldingsField|TestHasEthereumTokenNonzeroHoldings|TestShouldIncludeEthereumAddressToken' -count=1
• go test -tags unittest ./api -run 'TestHasEthereumTokenNonzeroHoldings|TestShouldIncludeEthereumAddressToken' -count=1
• go test -tags unittest ./server -run 'TestGetAddressQueryParamsParsesDocumentedAddressOptions' -count=1
• go test -tags unittest ./api -count=1
• go test -tags unittest ./server -count=1