Resolve implicit gas limits in transaction tests

## Problem

State and blockchain test fillers resolve an omitted transaction gas limit before signing. `TransactionTest` does not. As a result, transaction tests must provide an otherwise arbitrary gas limit or signing fails with `gas_limit must be set to sign a transaction`.

## Proposed change

Resolve omitted gas limits in `TransactionTest` before signing, using the shared implicit gas-limit behavior and a documented transaction-test gas budget. Keep explicit gas limits unchanged for boundary and invalid-transaction tests.

Add coverage for omitted and explicit limits, then remove redundant gas limits from transaction-test unit tests. Verify and document any fixture changes.