[WIP] [No QA] [Performance] Add API.writeWhenReady for deferred optimistic writes

[JakubKorytko]

Explanation of Change

Adds API.writeWhenReady(command, params, onyxData, barrier?): it defers a write - including its optimistic Onyx updates - until a readiness signal fires (default: the navigation transition), then delegates to the normal write() pipeline, leaving optimistic/success/failure handling, retries, and ordering unchanged. Foundational primitive; no call sites are migrated yet. Includes a safety timeout, an app-background flush, and unit tests.

Fixed Issues

$ #96010
PROPOSAL: N/A

Tests

This PR adds an API primitive with no call sites yet, so there is no user-facing flow to exercise; correctness is covered by unit tests.

Offline tests

N/A - no user-facing flow invokes this primitive yet. The deferral and app-background flush behavior (which persists a pending write before the process can be killed) is covered by the unit tests.

QA Steps

[No QA] - no code path invokes writeWhenReady yet, so there is nothing to verify on staging. Correctness is covered by the unit tests.

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
  • I added steps for the expected offline behavior in the Offline steps section
  • I added steps for Staging and/or Production testing in the QA steps section
  • I added steps to cover failure scenarios (i.e. verify an input displays the correct error message if the entered data is not correct)
  • 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).
• I included screenshots or videos for tests on all platforms
• I ran the tests on all platforms & verified they passed on:
  • Android: Native
  • Android: mWeb Chrome
  • iOS: Native
  • iOS: mWeb Safari
  • MacOS: Chrome / Safari
• I verified there are no console errors (if there's a console error not related to the PR, report it or open an issue for it to be fixed)
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  • I verified that any new or modified comments were clear, correct English, and explained "why" the code was doing something instead of only explaining "what" the code was doing.
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• If a new code pattern is added I verified it was agreed to be used by multiple Expensify engineers
• I followed the guidelines as stated in the Review Guidelines
• I tested other components that can be impacted by my changes (i.e. if the PR modifies a shared library or component like Avatar, I verified the components using Avatar are working as expected)
• If a new CSS style is added I verified that:
  • A similar style doesn't already exist
  • The style can't be created with an existing StyleUtils function (i.e. StyleUtils.getBackgroundAndBorderStyle(theme.componentBG))
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  • The assets are optimized and compressed (for SVG files, run npm run compress-svg)
  • The assets load correctly across all supported platforms.
• If the PR modifies code that runs when editing or sending messages, I tested and verified there is no unexpected behavior for all supported markdown - URLs, single line code, code blocks, quotes, headings, bold, strikethrough, and italic.
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• If the PR modifies a component related to any of the existing Storybook stories, I tested and verified all stories for that component are still working as expected.
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  • I added Design label and/or tagged @Expensify/design so the design team can review the changes.
• I added unit tests for any new feature or bug fix in this PR to help automatically prevent regressions in this user flow.
• If the main branch was merged into this PR after a review, I tested again and verified the outcome was still expected according to the Test steps.

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/API/index.ts	93.49% <95.16%> (+0.96%)	⬆️
... and 10 files with indirect coverage changes

[JakubKorytko]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Chef's kiss.

Reviewed commit: 323b61e8ea

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