fix(sdk-coin-ada): mark change outputs in explainTransaction

[bitgo-ai-agent-dev]

Summary

• ADA's explainTransaction() returned all outputs in a flat array with no change marker, causing intent verification to count change outputs as recipients and fire false-positive TransactionFailsIntentVerification alerts (~20 prod incidents)
• Add _changeAddress field to Transaction class with getter/setter
• Set changeAddress on the transaction from all three build paths in TransactionBuilder (processTokenBuild, processExplicitOutputsBuild, buildImplementation)
• In explainTransaction(), mark outputs whose address matches _changeAddress with change: true
• Add change?: boolean to the explainTransaction return type signature

Root Cause

sdk-coin-ada explainTransaction() set neither change: true nor isChange: true on change outputs. The intent verification parser reads output.change || output.isChange to filter change outputs — since ADA set neither, change outputs were counted as recipients, producing a recipients.count mismatch (intent: 1, transaction: 2).

A prior fix attempt (PR #58785, WAL-1464) was a no-op for ADA because it filtered on isChange: true which ADA never sets.

Test plan

• should mark the change output with change: true for a shelley send tx
• should mark the change output with change: true for a byron send tx
• should not set change on any output when no change address is set (consolidation)
• All existing sdk-coin-ada unit tests pass

Ticket: WCI-633

[linear-code]

WCI-633

[mohammadalfaiyazbitgo]

LGTM

[abhishekagrawal080]

Looks good