fix(client): isEdited missed edits fetched from the message box

[kyo5uke]

Description

TalkMessage.isEdited returns false for edited messages fetched from the
message box, while TalkMessage.updatedTime returns the edit time for the same
message. The two getters contradict each other.

isEdited (added in #225) only looked at contentMetadata.EDITED and the
updatedTime field. But a message returned by getRecentMessagesV2 carries
neither — only contentMetadata.UPDATED_TIME:

id: 630153925194416265
updatedTime (field 28) : absent
contentMetadata.EDITED : absent
contentMetadata.UPDATED_TIME : 1788432321630

updatedTime already reads UPDATED_TIME first, so it reports the edit
correctly; isEdited does not. Any chat history walked with
getRecentMessagesV2 / getPreviousMessagesV2 reports every edited message as
unedited.

isEdited now defers to updatedTime instead of repeating a narrower lookup,
so the two cannot drift apart again.

This also corrects a comment I got wrong in #225: it claimed updatedTime is
"only set on messages fetched from the message box". The message box does not
set that field at all.

Testing

• New regression test using the payload shape captured from a live
  getRecentMessagesV2 response. It fails before this change and passes after.
• deno test -A → 241 passed
• deno publish --dry-run → passes

Observed against my own account: a message edited from the LINE iOS app, then
re-fetched with getRecentMessagesV2.

Checklist

• Run tests
• Add jsdoc
• Test in your environment