fix: Clear convertedAmount when amount or currency is manually modified

[DebuggingMax]

Details

When a user manually edits an expense's amount or currency, the convertedAmount field was not being cleared. This caused incorrect currency conversion to be applied during report export.

Root Cause: The getUpdatedTransaction function in TransactionUtils sets modifiedAmount and modifiedCurrency when users edit expenses, but it did not clear convertedAmount. During export, the stale convertedAmount (calculated from the original currency/amount) would be used, leading to double conversion.

Example from the issue:

• User has a $45 USD Expensify subscription
• Credit card charged C$62.85 CAD
• User manually enters C$62.85 as the expense amount
• Before fix: Export shows C$85.63 (C$62.85 was treated as USD and re-converted to CAD)
• After fix: Export correctly shows C$62.85

Fixed Issues

$ #83451
$ #83451

Tests

Added unit tests in TransactionUtilsTest.ts to verify:

1. convertedAmount is cleared when currency is changed
2. convertedAmount is cleared when amount is manually modified

Offline tests

N/A - This change affects local state management that is then sent to the server.

QA Steps

1. Create an expense with a receipt that shows a different currency than manually entered
2. Manually edit the amount and/or currency to match your credit card charge
3. Export the report to CSV
4. Before: Exported amount was incorrectly re-converted
5. After: Exported amount matches the manually entered value

PR Author Checklist

• I linked the GitHub issue this PR fixes in the Fixed Issues section above
• I wrote clear testing steps that cover the changes made in this PR
• I added tests to verify changes
• I followed the code conventions described in the Style Guide
• I followed the guidelines for writing Good Component Structure in the Best Practices
• I validated my changes against standard platform devices using the checklist below

Screenshots/Videos

Web

N/A - Logic change only, no UI changes

Mobile Web - Chrome

N/A - Logic change only, no UI changes

Mobile Web - Safari

N/A - Logic change only, no UI changes

Desktop

N/A - Logic change only, no UI changes

iOS

N/A - Logic change only, no UI changes

Android

N/A - Logic change only, no UI changes

[melvin-bot]

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Use null so convertedAmount is actually removed

getUpdatedTransaction() sets convertedAmount to undefined, but this object is later written with Onyx.METHOD.MERGE (src/libs/actions/IOU/index.ts, around the optimistic transaction merge), and this codebase explicitly treats undefined as a no-op for Onyx.merge (see comments in src/pages/workspace/companyCards/addNew/StatementCloseDateStep.tsx:33 and src/libs/actions/TravelInvoicing.ts:74). In the amount/currency edit flow, that means the stale convertedAmount value is preserved instead of cleared, so the export double-conversion issue this commit targets can still occur.

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