fix(ai): require reasoning fields for deepseek assistants - #45075

Summary

• add the public requireReasoning model compatibility option and expose it in generated client types
• infer required assistant reasoning from DeepSeek model IDs, the DeepSeek provider, or DeepSeek API endpoints while honoring explicit overrides
• include an empty configured reasoning field on assistant text and tool-call messages when required, preserving custom field names and existing explicit reasoningField behavior
• cover model/provider/endpoint detection, custom reasoning fields, explicit opt-out, public schema decoding, and model resolution

Test plan

• bun test test/provider/openai-chat.test.ts test/provider/openai-compatible-chat.test.ts && bun typecheck from packages/ai
• bun test test/model.test.ts && bun typecheck from packages/schema
• bun test test/model-resolver.test.ts && bun typecheck from packages/core
• bun run generate && bun typecheck from packages/client
• broader bun test test/provider currently encounters unrelated existing Anthropic/OpenRouter recorded-fixture and OpenRouter expectation failures