feat: Derive fiat order `sourceAmount` from on-chain tx data #8694

Bugbot Review

Bugbot Analysis Progress (7m 36s elapsed)

✅ Gathered PR context (3s)
✅ Completed bug detection (7m 29s)
✅ Posted analysis results (5s)

Final Result: Bugbot completed review and found 1 potential issue.

Request ID: serverGenReqId_85d3a50a-d2fc-4b25-8dd5-0c15b7113f93

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, have a team admin enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 0a7e13c. Configure here.}