Handle failed optimistic user messages

[SaravanakumarR2018]

Motivation

• Preserve user-submitted messages in the UI when a send fails by marking optimistic messages as failed instead of removing them, improving error visibility and preserving user intent.

Description

• Add src/frontend/src/utils/optimisticMessageUtils.ts with markOptimisticMessageFailed to set optimistic: false and failed: true on a message object.
• Update the playground send flow in src/frontend/src/modals/IOModal/playground-modal.tsx to create an optimistic user message (using uuid) and addMessage it before sending, and to call markOptimisticMessageFailed on catch to preserve the message state on error.
• Change addMessage logic in src/frontend/src/stores/messagesStore.ts to replace an existing optimistic/failed user message when the real user message arrives by matching sender, session_id, and text, preventing duplicate messages.
• Remove an unused setIsBuilding selector and update sendMessage dependencies accordingly.

Testing

• No automated tests were run.

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Codex Task

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[chatgpt-codex-connector]

Import existing message type to avoid build failure

The new utility imports MessageType from @/types/messages, but that module only exports Message (see src/frontend/src/types/messages/index.ts). Because this is a value import (not import type) and the project uses isolatedModules/noEmit, bundlers like Vite/esbuild will treat it as a runtime import and error with “export not found,” and TypeScript will also fail the build. Use the existing Message type (ideally via import type { Message }) or add a real MessageType export.

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