Fix stale isValidatingMultipleFiles state in file validation error modal

[MobileMage]

Explanation of Change

When dropping >30 files for the first time, the error modal showed a "Close" button instead of "Continue" + "Cancel". This happened because setErrorAndOpenModal read isValidatingMultipleFiles from React state, which was still false at the time of the call — setIsValidatingMultipleFiles(true) was called in the same synchronous block but React hadn't re-rendered yet.

The fix removes the setErrorAndOpenModal helper and inlines the error object construction using validationState.isValidatingMultipleFiles — a synchronously computed local value that is always correct. Also fixed the same stale-state bug in two image-resize error paths inside validateAndResizeFiles.

Fixed Issues

$ #87470
PROPOSAL: #87470 (comment)

Tests

1. Open any chat or request money flow where you can attach files
2. Select or drag-and-drop more than 30 files at once (mix of images, PDFs, etc.)
3. Verify the error modal shows "Continue" and "Cancel" buttons (not just "Close")
4. Tap "Continue" and verify the first 30 files proceed to validation
5. Repeat step 2 — verify the modal consistently shows the correct buttons on every attempt, not just the second time

• Verify that no errors appear in the JS console

Offline tests

N/A — file validation is a client-side operation that does not depend on network state.

QA Steps

1. Open any chat
2. Drag-and-drop more than 30 files into the composer
3. Verify the error modal shows "Continue" and "Cancel" buttons
4. Tap "Continue" and verify files are processed
5. Repeat step 2 and verify the modal still shows the correct buttons

• Verify that no errors appear in the JS console and copy to clipboard
• [ ]

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

[melvin-bot]

@eh2077 Please copy/paste the Reviewer Checklist from here into a new comment on this PR and complete it. If you have the K2 extension, you can simply click: [this button]