fix(ai): align chat assistant content

[opencode-agent]

Summary

• serialize reasoning-only assistant messages with content: "" while retaining content: null for tool-call messages
• concatenate assistant text parts without adding separators
• finalize accumulated tool calls and emit terminal events when compatible streams end cleanly without finish_reason
• preserve reasoning deltas that arrive after text has started instead of silently dropping them
• align native OpenAI Chat request lowering and stream handling with the AI SDK OpenAI and OpenAI-compatible adapters

Testing

• bun typecheck in packages/ai
• focused OpenAI Chat regression tests: 5 passing
• full openai-chat.test.ts: 48 passing; one unrelated existing Azure URL assertion fails because the generated URL contains a duplicate /v1

Fixes #41165

Requested by: @rekram1-node (Aiden Cline via Slack)