fix: allow addSibling to target the root message

[RudrenduPaul]

Closes #2386, Closes #2387

What this PR does

addSibling in src/lib/utils/tree/addSibling.ts threw an error whenever
the sibling target was the conversation's root message:

Error: The sibling message is the root message, therefore we can't add a sibling

This restriction was unnecessary. A sibling of the root message is just
another top-level message with an empty ancestors array, and the rest of
the function already handles that case correctly.

Root cause

The guard clause explicitly threw when sibling.ancestors was empty or
undefined:

if (!sibling.ancestors || sibling.ancestors?.length === 0) {
  throw new Error("The sibling message is the root message, therefore we can't add a sibling");
}

Change

Removed the guard clause. With it gone, the existing logic already handles
the root case correctly:

• sibling.ancestors is [], so the new message is pushed with
  ancestors: [] (same as the root).
• nearestAncestorId resolves to sibling.ancestors[sibling.ancestors.length - 1],
  i.e. [][-1], which is undefined.
• conv.messages.find(m => m.id === nearestAncestorId) returns undefined
  since no message has id === undefined.
• The if (nearestAncestor) block is therefore skipped, which is exactly
  right: there is no parent node whose children array needs updating.

The non-root path is completely unaffected — the removed guard only ever
fired when ancestors was empty/falsy, so every other call path behaves
identically to before.

Also updated addSibling.spec.ts: the existing test asserted the old
throwing behavior (with a // TODO: This behaviour should be fixed comment
already acknowledging it needed to change). Replaced it with a test that
confirms addSibling now succeeds when targeting the root message and
returns a new message with the same (empty) ancestors as the root.

Verification

• Traced the full function logic by hand for the root-message case to
  confirm no other code path re-throws or crashes once the guard is gone.
• npx prettier --check and npx eslint both pass clean on the changed files.
• Ran the full addSibling.spec.ts suite: 8/8 tests pass, including the
  updated root-message test and the pre-existing "should add a sibling to
  a message" test (confirms non-root behavior is unchanged).

Note: Claude Code was used to assist in drafting this fix. All changes were reviewed by the submitter.

[chatgpt-codex-connector]

💡 Codex Review

chat-ui/src/lib/utils/tree/addSibling.ts

Lines 27 to 28 in a341dde

	const nearestAncestorId = sibling.ancestors[sibling.ancestors.length - 1];
	const nearestAncestor = conv.messages.find((m) => m.id === nearestAncestorId);

Record root-level siblings in the branch index

When the target is the root, sibling.ancestors is empty, so nearestAncestorId becomes undefined and no children array is updated. The branch picker is built exclusively from message.children (createMessagesAlternatives in src/routes/conversation/[id]/+page.svelte lines 116-123, then matched in ChatWindow.svelte line 620), so retrying or editing the first user message creates a second top-level message that is not present in any alternatives list; after this succeeds, users cannot navigate between the original root branch and the retried root branch. Please also represent root siblings in the branch data, or teach the UI to include messages with empty ancestors.

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[RudrenduPaul]

Thanks @chatgpt-codex-connector for catching this — confirmed and fixed in the latest commit.

Root-level siblings had no parent to record them in a children array, so they were created but never appeared in any messagesAlternatives group, making them unreachable from the branch switcher UI. createMessagesAlternatives in +page.svelte now also emits a root-level alternatives group when more than one top-level message exists, so ChatMessage's existing alternatives={messagesAlternatives.find((a) => a.includes(message.id))} lookup picks it up with no other UI changes needed.

While verifying this, svelte-check also caught that removing the original guard clause in addSibling.ts had silently dropped a TypeScript narrowing check on sibling.ancestors (2 new type errors) — fixed with an explicit ?? [] fallback. All 8 existing tests still pass.

[chatgpt-codex-connector]

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