Fix participant picker flashing on Create expense when Submissions are disabled

[MelvinBot]

Explanation of Change

Fixes the deploy blocker where the participant (recipient) list briefly appears and then disappears when opening FAB → Create expense on a workspace with Submissions disabled.

Root cause: After PR #96228 removed the NEW_MANUAL_EXPENSE_FLOW beta and made the embedded-confirmation + auto-opening participant picker the default, the auto-open decision (shouldAutoOpenParticipantPicker) is derived purely from whether defaultParticipants is currently populated — it has no "still loading" state. On the first render(s) the Onyx sources that resolve the default destination haven't hydrated yet, so defaultParticipants is momentarily empty and the picker auto-opens. A beat later the default resolves (via the self‑DM branch when auto‑reporting/Submissions is off — see useDefaultParticipants.ts), the auto‑assign effect fires and the picker closes — the visible flash. It only reproduces with Submissions disabled because the self‑DM branch settles a render or two behind the policy‑expense‑chat branch used when Submissions are enabled.

Fix: useDefaultParticipants now also reports whether the Onyx data backing the resolution is still hydrating (isLoading), computed from the load status of the collections/keys the resolution reads (POLICY, REPORT, and the billing NVPs). All the lagging sub-hooks (useDefaultExpensePolicy, useSelfDMReport) and getPolicyExpenseChat derive from those same POLICY/REPORT collections, so isLoading stays true precisely during the hydration window. IOURequestStepConfirmation now keeps shouldAutoOpenParticipantPicker false while isLoading is true, so an "empty because not resolved yet" state no longer flashes the picker. Once the data settles, a genuinely empty default still opens the picker as before. The status-only collection subscriptions use a constant selector so they don't add value-driven re-renders.

⚠️ Note for the co-author: MelvinBot could not execute the local AI checks (lint/typecheck/jest/react-compiler) in its environment, so those are being relied upon via CI here. Please run them locally and QA the exact repro (workspace with Submissions disabled → FAB → Create expense → the participant list must not appear at all), plus a regression pass on the normal create-expense flow (Submissions enabled, workspace-chat entry point, and the genuine no-default case where the picker should still auto-open) before marking ready.

Fixed Issues

$ #96558
PROPOSAL:

Tests

// TODO: The human co-author must fill out the tests you ran before marking this PR as "ready for review". Suggested: 1) Create a workspace and disable Submissions in Workflows. 2) Open FAB → Create expense. 3) Verify the participant list does NOT appear (no flash). 4) Regression: with Submissions enabled and from a workspace chat, verify create expense still works; verify the picker still auto-opens when there is genuinely no default participant.

• Verify that no errors appear in the JS console

Offline tests

QA Steps

// TODO: The human co-author must fill out the QA tests you ran before marking this PR as "ready for review".

• 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
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  • I added steps for the expected offline behavior in the Offline steps section
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  • I turned off my network connection and tested it while offline to ensure it matches the expected behavior (i.e. verify the default avatar icon is displayed if app is offline)
  • I tested this PR with a High Traffic account against the staging or production API to ensure there are no regressions (e.g. long loading states that impact usability).
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Screenshots/Videos

Android: Native

Android: mWeb Chrome

iOS: Native

iOS: mWeb Safari

MacOS: Chrome / Safari

[codecov]

Codecov Report

❌ Looks like you've decreased code coverage for some files. Please write tests to increase, or at least maintain, the existing level of code coverage. See our documentation here for how to interpret this table.

Files with missing lines	Coverage Δ
src/hooks/useDefaultParticipants.ts	88.88% <100.00%> (+2.52%)	⬆️
src/hooks/useResetIOUType.ts	97.29% <100.00%> (ø)
...es/iou/request/step/IOURequestStepConfirmation.tsx	64.16% <66.66%> (-0.09%)	⬇️
... and 10 files with indirect coverage changes