[Agent Builder] Attachment inline rendering and UI actions

[chrisbmar]

closes https://github.com/elastic/search-team/issues/12939

This PR implements the plumbing for the acceptance criteria below, it does not include any UI.

You can see in the following video that the attachment and UI definition is reaching the chat response:

Screen.Recording.2026-02-16.at.17.46.33.mov

Implement:

• the ability for solutions to define a custom render function in the attachment UI registry
• the ability solutions to define UI actions in the attachment UI registry

[joemcelroy]

not blocking but we should consider pushing this up into agentBuilderServices hook

[chrisbmar]

we technically don't even need it considering the agent doesn't have any prompt to emit the custom XML tag... so I could remove it for now? This was basically just a guard in case the agent emitted the custom XML tag for attachment_renderer

[joemcelroy]

yeh we can remove for now.

[chrisbmar]

waiting forever for CI to pass, so will merge this PR as is once CI is done and in the canvas-mode PR I'll remove this check 🙏🏽

[ppisljar]

LGtM

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 42eef38

Metrics [docs]

Module Count

Fewer modules leads to a faster build time

id	before	after	diff
agentBuilder	1714	1716	+2

Public APIs missing comments

Total count of every public API that lacks a comment. Target amount is 0. Run node scripts/build_api_docs --plugin [yourplugin] --stats comments for more detailed information.

id	before	after	diff
@kbn/agent-builder-browser	15	17	+2

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
agentBuilder	574.0KB	576.1KB	+2.1KB

Public APIs missing exports

Total count of every type that is part of your API that should be exported but is not. This will cause broken links in the API documentation system. Target amount is 0. Run node scripts/build_api_docs --plugin [yourplugin] --stats exports for more detailed information.

id	before	after	diff
@kbn/agent-builder-browser	0	3	+3

Page load bundle

Size of the bundles that are downloaded on every page load. Target size is below 100kb

id	before	after	diff
agentBuilder	87.3KB	87.3KB	+45.0B

Unknown metric groups

API count

id	before	after	diff
@kbn/agent-builder-browser	33	37	+4

History

• 💛 Build #397582 was flaky 806ae0c

cc @chrisbmar