fix: keep subagent spinner and footer live while a subagent runs

[MattPua]

Problem

When a sub-agent (Task/Agent tool call) is running, the conversation UI sometimes looked done while the sub-agent was still working. Two faces:

1. The sub-agent row's spinner stopped once the parent turn completed (or once the spawn tool call settled), even though the sub-agent kept running.
2. The footer's "Generating…" indicator stopped for the same reason.

This affects both the Claude and Codex adapters.

Changes

Derive live state from the sub-agent's own nested child tool calls instead of the parent turn's turnComplete / the spawn's own status:

• SubagentToolView keeps its spinner while any child tool call is non-terminal, even after the parent turn completes.
• New-thread ToolGroup / chip summary stay "Using" while a sub-agent in the group has active children.
• Both footers (legacy ConversationView and ChatThreadFooter) keep the generating indicator alive via a new conversationHasActiveSubagent helper.

Display-only: the prompt-lifecycle gates (message queue / steer) still read the raw isPromptPending, so a follow-up message sends rather than queuing while a detached sub-agent finishes.

Also restores Codex's wait collab-agent event ("Waiting for subagents" row) that PR #3522 had hidden alongside closeAgent; closeAgent stays hidden as lifecycle noise.

How did you test this?

• New unit tests in buildConversationItems.test.ts for conversationHasActiveSubagent (active child, detached sub-agent after turn end, clears on settle, ignores non-subagent).
• New SubagentToolView.test.tsx asserting the spinner stays up while a child is in_progress with turnComplete=true, and clears when settled.
• pnpm --filter @posthog/ui exec vitest run src/features/sessions/ (467 passing)
• pnpm --filter agent exec vitest run src/adapters/codex-app-server/mapping.test.ts (39 passing)
• pnpm --filter @posthog/ui typecheck, biome lint on changed files

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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Created with PostHog Code

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 880a27a.}