test: cover isMessageSequential grouping rules

[ishan-one8]

Cover isMessageSequential grouping rules with unit tests

Acceptance Criteria fulfillment

• Adds packages/layout_editor/src/lib/isMessageSequential.test.js, covering every branch of the grouping rules
• Test-only — no source changes (git diff on the source is empty)
• Follows the existing conventions in that folder: node:test, node:assert/strict, flat test() calls
• Full layout_editor suite passes locally — 25/25 (17 existing + 8 new)

Fixes #1373

Video/Screenshots

Not applicable — no visible change, test file only.

PR Test Details

yarn test in packages/layout_editor (node --test src/lib/*.test.js) → 25 pass, 0 fail.

Behaviours now covered:

• no previous message
• same user inside the grouping range (the actual grouping case)
• different users
• a gap wider than the grouping range
• system messages (t set on either message)
• groupable: false
• differing alias
• a day boundary inside the grouping range

These are not tests that pass when the behaviour is broken. Removing the day-boundary guard from the source (return isTimeDiffSmall && !isMessageNewDay → return isTimeDiffSmall) makes the last one fail:

✖ does not group across a day boundary even inside the grouping range
  AssertionError [ERR_ASSERTION]: Expected values to be strictly equal:
  true !== false

The source was reverted afterwards — this PR touches no source file.

Note: the day-boundary test uses local-time timestamps rather than UTC Z strings on purpose, since isSameDay compares local calendar days.