fix(deepseek): update deprecated models to v4

[AkhilTrivediX]

Summary

• Fixes Deepseek models deprecation #6667 by replacing the deprecated DeepSeek models (deepseek-chat and deepseek-reasoner) with their new v4 counterparts (deepseek-v4-flash and deepseek-v4-pro).
• Updates the default model configuration in the DeepSeek chat node.
• Updates the models.json definitions and input/output costs according to current DeepSeek API pricing.

Testing

• Ran pnpm install and pnpm build across the workspace locally; all tasks completed successfully.
• Verified Deepseek.ts has the updated default model.

[gemini-code-assist]

Caution

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