Add regression coverage for reserved message property detection

## Problem
The current test suite does not cover reserved message property detection.

## Expected behavior
- A focused Vitest case locks the existing behavior.
- The targeted test file passes and fails if the covered behavior regresses.

## Scope
Only add regression coverage for $(@{Slug=reserved-message-property; Focus=reserved message property detection; PrTitle=test(message-properties): cover reserved property detection; TestFile=web/src/utils/messagePayloadPreview.test.ts; Mode=existing; Append=describe('message reserved property detection', () => {
it('recognizes reserved message properties case-insensitively after trimming', () => {
expect(isReservedMessageProperty('tags')).toBe(true);
expect(isReservedMessageProperty(' UNIQ_KEY ')).toBe(true);
expect(isReservedMessageProperty('tenant')).toBe(false);
});
});}.TestFile). No runtime behavior changes.

## Acceptance criteria
- [ ] The targeted Vitest file passes.
- [ ] git diff --check is clean.

Start with web/src/utils/messagePayloadPreview.test.ts and the existing isReservedMessageProperty coverage. Run the targeted Vitest file, then add the specified case for trimmed, case-insensitive reserved properties; done means the test passes and git diff --check is clean.

Written by the indexing model from the issue text.