chatapi: remove assistant-mode execution branches

[Saby-Bishops]

Motivation

• Assistant-only execution paths and related config are deprecated and need removal to simplify chat execution to completion-based flows.
• The assistant/thread/run flow added complexity and relied on Assistant-specific config that is no longer required.

Description

• Removed chatapi/src/utils/chat-assistant.utils.ts and deleted assistant-specific runtime/thread/run streaming logic, so both streaming and non-streaming flows call provider chat completions directly.
• Removed the assistant field from aiChat, chat, and chatNoSave signatures and updated all call sites to stop accepting/forwarding the assistant flag (chatapi/src/utils/chat.utils.ts, chatapi/src/services/chat.service.ts, chatapi/src/index.ts).
• Dropped assistant export and config population from chatapi/src/config/ai-providers.config.ts and removed the assistant type from chatapi/src/models/chat.model.ts.
• Cleaned up imports and unused parameters in helper modules (chatapi/src/utils/chat-helpers.utils.ts).

Testing

• Ran cd chatapi && npm run build and the TypeScript build completed successfully.
• Ran cd chatapi && npm run lint and ESLint reported no errors (one prior unused-var warning was resolved).

Note: This change removes a user-visible AI mode; obtain PM/owner sign-off before deployment due to high product risk.

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Codex Task