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

Setup — produce two eligible distance expenses

Both expenses must be distance requests and neither may be on an IOU report. They do not need a workspace — two unreported (personal) distance expenses are eligible.

1. Create a distance expense via the Odometer tab — enter start/end readings and attach a receipt. (If the Odometer tab isn't available on a personal account, create both expenses on a workspace that has odometer distance enabled.)
2. Create a distance expense via the Map tab — set a route with at least two waypoints and attach a receipt.

Merge — finishing on the map expense

3. Open the odometer expense, tap the header's More (⋯) menu, and select Merge.
   • The map expense should appear in the eligible list. If you see "No eligible expenses found" instead, the setup is wrong: confirm both expenses are distance requests, neither is on an IOU report, and neither is still scanning/pending.
4. Select the map expense to merge with.
5. On the Details review page, for the Merchant field, tap the odometer expense, then the map expense, then the odometer expense again (toggle back and forth), finishing on the map expense.
6. Tap Continue and confirm the merge.
7. 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).

Merge — finishing on the odometer expense

8. Repeat steps 3–6, but finish the Merchant toggle on the odometer expense.
9. 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
• I wrote clear testing steps that cover the changes made in this PR
  • I added steps for local testing in the Tests section
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• I included screenshots or videos for tests on all platforms
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  • Android: Native
  • Android: mWeb Chrome
  • iOS: Native
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  • MacOS: Chrome / Safari
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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

[MelvinBot]

🤖 Ran the tests against e429098 (jules-fixOdometerMapMergeLeak). The PR's unit tests and all core CI checks pass.

Check	Result
Unit tests (MergeTransactionTest.ts, MergeTransactionUtilsTest.ts)	✅ 104 passed, 0 failed
typecheck (tsc)	✅ Passed
ESLint (changed files)	✅ Passed
Prettier (--check)	✅ Passed
React Compiler check-changed	⚠️ Could not run — see below

Details

• Unit tests — ran the two suites changed in this PR: 2 passed, 2 total / 104 tests passed.
• typecheck — npm run typecheck (tsc) completed clean.
• ESLint — ran ./scripts/lint.sh directly on the 4 changed files (the lint-changed wrapper can't compute a merge-base in this shallow checkout, so I targeted the files explicitly). No violations.
• Prettier — --check reports all matched files use Prettier code style.
• React Compiler check-changed — couldn't run in this environment: the script resolves the base branch via origin/<GITHUB_BASE_REF>, which is empty here, so it errors on git rev-parse origin/. This is an environment limitation of the runner, not a problem with the PR. It will still run normally in CI.

Files tested: src/libs/MergeTransactionUtils.ts, src/pages/TransactionMerge/DetailsReviewPage.tsx, plus the two test files above.

[MelvinBot]

⏱️ Ran out of time before I could finish. This usually means the task was too complex or I got stuck exploring. Please try again with a more specific request or post in #melvinbot-ai.

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