[Fix] Discord Fast replies fail to attach selected images

[roomote-roomote]

​Opened on behalf of @mrubens. Follow up by mentioning @roomote-roomote, in the web UI, or in Slack.

What changed

• Fast sends selected imageArtifactIds as native Discord image embeds in direct Discord replies and replies on existing Fast sessions.
• Resolve selected images through the current Fast session before delivery. Deferred interaction replies edit the first ten embeds and post any remaining image batches; retry replacements also preserve embeds or repost through the image-capable path when they exceed edit limits.
• Added focused regressions for artifact resolution and delivery, deferred interactions, and retry replacements.

Why this change was made

The Discord Fast adapter accepted a reply carrying selected image IDs but posted only its text, so Fast could claim a screenshot was attached when Discord received no image. Deferred Discord interactions also have a ten-embed per-message limit; larger selections must send additional batches rather than fail the whole reply.

Impact

Discord Fast replies now deliver selected task screenshots instead of silently dropping them, including retry replacement and deferred-interaction overflow paths. The fix does not overlap #3201's artifact-upload notification changes. Focused automated tests cover the Discord payload and session-scoped artifact resolution; visual proof capture timed out, so no screenshots are included.

[roomote-community]

No code issues found. See task

• Fast can claim an earlier task image is attached on Discord even though its adapter drops imageArtifactIds.
• Deferred Discord interaction replies with more than ten selected image artifacts fail instead of delivering image batches.

_{Reviewed 65bbfde}

[roomote-roomote]

Resolved both Discord image-delivery findings in [0ad6b80](https://github.com/RooCodeInc/Roomote/commit/0ad6b809) and [65bbfde](https://github.com/RooCodeInc/Roomote/commit/65bbfde0). Fast now resolves selected image IDs against the current session, posts Discord embeds on direct and existing-session replies, and batches deferred-interaction overflow beyond ten images. Both review threads are resolved, and the latest Roomote code review check passes with no code issues. Focused API/SDK/Discord-provider tests, typechecks, pre-push checks, and CI passed. Proof handoff: blocked, `proof capture timed out`; no visual proof is included.