Fix stale fields leaking when merging odometer and map distance expenses

[Julesssss]

Explanation of Change

Merging an odometer distance expense with a map distance expense produced a corrupted result — leaked odometer readings, the wrong route, and a combined receipt — because the front end accumulated both modalities' fields in the merge transaction instead of replacing them on each merchant selection.

Two distinct leaks, both fixed by making a merchant selection fully own the distance bundle:

• Odometer readings (getMergeFieldUpdatedValues, MergeTransactionUtils.ts): odometer fields were only set when the selected expense was itself an odometer request, so selecting a map merchant left the previous odometer readings untouched — Onyx.merge ignores omitted keys. They are now explicitly nulled for any non-odometer selection.
• receipt and customUnit (handleSelect, DetailsReviewPage.tsx): these are objects, and setMergeTransactionKey uses Onyx.merge, which deep-extends them and keeps stale sub-keys from the previously selected expense. They are now nulled first in the same synchronous batch so the new value fully replaces rather than merges.

Fixed Issues

$ #91794
PROPOSAL:

Tests

1. Create an odometer distance expense — enter start/end readings and attach a receipt.
2. Create a map distance expense — set a route with waypoints and attach a receipt.
3. From one of the expenses, start the Merge expenses flow and select the other expense to merge with.
4. On the details review page, for the Merchant field, tap the odometer expense, then tap the map expense, then the odometer expense again (toggle back and forth), finishing on the map expense.
5. Tap Continue and confirm the merge.
6. Verify the merged expense is a clean map distance expense: it shows the map route/waypoints, shows no odometer readings, and the receipt is the map expense's receipt only (not a combined image).
7. Repeat steps 3–5 but finish the Merchant toggle on the odometer expense.
8. Verify the merged expense shows the odometer readings, no map route/waypoints, and the odometer receipt only.

• Verify that no errors appear in the JS console

Offline tests

QA Steps

Same as tests.

• Verify that no errors appear in the JS console

PR Author Checklist

• I linked the correct issue in the ### Fixed Issues section above
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Screenshots/Videos

Android: Native

Android: mWeb Chrome

iOS: Native

iOS: mWeb Safari

MacOS: Chrome / Safari

[codecov]

Codecov Report

✅ Changes either increased or maintained existing code coverage, great job!

Files with missing lines	Coverage Δ
src/libs/MergeTransactionUtils.ts	81.93% <100.00%> (+2.10%)	⬆️
src/pages/TransactionMerge/DetailsReviewPage.tsx	0.00% <0.00%> (ø)
... and 101 files with indirect coverage changes