## Summary
Outbound email messages never populate
`content_attributes.email.subject`. The subject lives only on the parent
conversation's `additional_attributes.mail_subject`. The search index
doc built by `Messages::SearchDataPresenter` pulls from the
message-level field only, so outbound email subjects are unsearchable
for accounts on the advanced_search (Elasticsearch) path.
This change makes the presenter fall back to
`conversation.additional_attributes.mail_subject` when the message-level
subject is blank. Inbound email messages keep their existing behavior
(message-level subject takes precedence).
Closes https://linear.app/chatwoot/issue/CW-6877
## What changes
- `Messages::SearchDataPresenter#content_attributes_data` now falls back
to the conversation's `mail_subject` when the message-level subject is
blank.
- Added specs covering the fallback, precedence, and the neither-set
case.
## What does not change
- No schema changes, no migrations, no backfill of existing OpenSearch
documents.
- Searchkick's `after_commit :reindex_for_search` on `Message` will pick
up the new field for all newly created or updated messages via the
existing indexing path.
- Postgres search path (free accounts, fallback) is untouched. Broader
subject search for those users is a separate follow-up.
