[Agent Builder] Persist read and status for conversations

[petrkrejcik]

Closes https://github.com/elastic/search-team/issues/14515

Summary

Backend foundation for the Chat status epic (#14513). Adds two new conversation-level fields the frontend needs to render sidebar status chat icons:

• read: boolean
  • tracks whether the user has seen the latest content
  • set to false on each round write to a conversation (e.g. renaming doesn't affect read)
  • not returned from API for legacy conversations that pre-date this change
• status: ConversationRoundStatus
  • denormalized from the last round's status so the list endpoint doesn't need to load rounds
  • stored on each round write
  • not returned from API for legacy conversations that pre-date this change

Decisions

• the naming status (new) vs state (existing). I considered different name like last_round_status but that reveals an implementation details that might change in the future, so I decided to keep the same existing name (even though a bit confusing because of the existence of state)
• I also considered placing status into state but as state is not indexed, it's a no go

Checklist

Check the PR satisfies following conditions.

Reviewers should verify this PR satisfies this list as well.

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the docker list
• This was checked for breaking HTTP API changes, and any breaking changes have been approved by the breaking-change committee. The release_note:breaking label should be applied in these situations.
• Flaky Test Runner was used on any tests changed
• The PR description includes the appropriate Release Notes section, and the correct release_note:* label is applied per the guidelines
• Review the backport guidelines and apply applicable backport:* labels.

Identify risks

• dynamic: strict mapping update. .chat-conversations is dynamic: strict; new fields must be added to the mapping before any write succeeds. Severity: Low. Mitigation: Check with peers how this work.

[kibanamachine]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 18a4fd1
• Build duration: 64 mins

Failed CI Steps

• FTR Configs #69
• FTR Configs #76
• FTR Configs #145
• FTR Configs #161

Test Failures

• [job] [logs] FTR Configs #145 / alerting api integration security and spaces enabled - Group 2 Connectors Bedrock executor execution successful response simulator should send a stringified JSON object with latest body

Metrics [docs]

✅ unchanged

History

• 💚 Build #450008 succeeded bff36c5

cc @petrkrejcik

[chrisbmar]

LGTM 🚀 let's wait for @pgayvallet review before merging