fix(onchain): remove SorobanOnchainAdapter stub that fabricated success (#449)

SorobanOnchainAdapter posted a hand-rolled {contractId, method, args}
JSON blob to simulateTransaction/sendTransaction — not a Soroban
envelope — and returned status 'success' with an empty transactionHash
for every mutation, silently corrupting the audit trail. Delete it and
its stale coverage-baseline entry, keeping SorobanAdapter as the single
canonical adapter, and add a regression spec proving the adapter
submits real TransactionBuilder-built envelopes and only reports
success with a confirmed non-empty hash.

Author: lovesmilesmall-hue