Fix ACP session recovery

[wgqqqqq]

What changed

• Keep ACP chat sessions pinned to their acp:<client> agent type so the input capsule close action cannot silently switch them back to the normal agentic path.
• Route cancel actions for ACP sessions through cancel_acp_dialog_turn with workspace and remote identifiers.
• Add request IDs and timeout cleanup for ACP create/restore loading indicators, and time out ACP session option restores so the UI does not leave restoring session stuck on screen.
• Add tests for ACP session detection and fallback behavior.

Why

Closing the ACP capsule prevented the same chat from returning to ACP mode, and switching back to a long-running ACP session could leave the UI stuck on restoring session. The root cause was frontend state losing the ACP agent type and pending restore UI state lacking a robust cleanup path.

Validation

• pnpm --dir src/web-ui run test:run src/flow_chat/utils/acpSession.test.ts src/flow_chat/utils/chatInputMode.test.ts
• pnpm run type-check:web
• git diff --check