fix(transaction): use arg.toObject() in Transaction#fromObject

[exodus-neo]

Summary

• Transaction.prototype.fromObject called transaction.toObject() while the local transaction variable was still undefined, so calling fromObject with a Transaction instance always threw TypeError: Cannot read properties of undefined (reading 'toObject').
• Fixed by reading from the arg parameter (arg.toObject()), which holds the Transaction instance in that branch — consistent with the arg instanceof Transaction guard and upstream bitcore-lib behavior.
• Added a regression test for fromObject with a Transaction instance, a path that had no coverage in the active test suite (which is why the typo shipped).

Details

The buggy branch is only reachable when fromObject is called directly with a Transaction instance (e.g. new Transaction().fromObject(otherTx)). The constructor routes Transaction instances to shallowCopy instead, which is why this went unnoticed. The bug is long-standing (inherited from the upstream fromObject history), not a recently introduced regression, so no Fixes regression from #N reference is included.

Verification

• npm test: 451 passing (was 450; +1 new regression test), 0 failing.
• The new test reproduces the exact TypeError before the fix (red) and passes after (green).
• npm run lint could not run in this environment: gulp/jshint are not installed (absent from devDependencies). The change is a single-token edit consistent with the surrounding style, and both changed files pass node --check.

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Origin: https://exodusio.slack.com/archives/C0B3VPDD5S9/p1780580205114779