[Bug]: Antigravity does not resume turns on subagent/background task completion #30082

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/server

Steps to reproduce

1. Start T3 Code and open a session with the Antigravity provider (gemini-3.8-flash-high).
2. Run a task that delegates work to a subagent (delegate_task) or executes background operations.
3. Wait for the subagent or background task to complete.

Expected behavior

The parent Antigravity turn receives the continuation delivery and resumes processing the subagent results.

Actual behavior

The parent session stalls indefinitely in "Waiting on background task..." because deferFinalizeForBackgroundWork and enablePostSettleContinuation are omitted from the Antigravity ACP adapter flavor. Additionally, assistant streaming paragraphs were held until full turn completion if no markdown boundary followed.

Impact

Major degradation or frequent failure

Version or commit

main @ cefb544

Environment

macOS, Antigravity ACP runtime 1.4.5, gemini-3.8-flash-high

Workaround

Manually interrupting or sending new prompts after subagents finish.

Related issues

• Related to Assistant streaming in 'paragraph' mode withholds final paragraph while turn remains open on background tool calls, and steering a delegated wake run disposes sibling app_owned subagent wakeups #28175 (Assistant streaming paragraph withholds text while turn remains open / subagent wakeups)
• Related to Antigravity ACP (Gemini 4 Argon): Streaming text gets stuck in running/streaming state & thoughts are never streamed #28172 (Antigravity streaming stuck in running/streaming state)
• Related to [Bug]: Antigravity orchestrator hangs indefinitely after delegated tasks complete (missing post-settle continuation) #17412 (Antigravity orchestrator hangs indefinitely after delegated tasks complete)