Show optimistic user messages immediately in playground

[SaravanakumarR2018]

Motivation

• Users reported that their typed question in the playground was not visible until the server response completed, creating a jarring delay; the goal is to make the input appear immediately.
• Provide optimistic UI for user messages so the chat feels responsive while the flow is being built and executed.

Description

• Insert optimistic user messages before the build runs by adding a temporary message in IOModal's sendMessage flow (file src/frontend/src/modals/IOModal/playground-modal.tsx) using ShortUniqueId and useMessagesStore.addMessage with a properties.optimistic flag.
• Update message reconciliation in useMessagesStore.addMessage (file src/frontend/src/stores/messagesStore.ts) to replace an existing optimistic message with the authoritative server message when it arrives by matching flow_id, session_id, text, and sender, preventing duplicate entries.

Testing

• No automated tests were executed as part of this change (not run).

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Codex Task