fix(ai): parse compatible reasoning deltas

[opencode-agent]

Summary

• parse streamed reasoning_content, reasoning, reasoning_text, and structured reasoning_details
• merge incremental structured detail chunks by identity so text and signatures replay as one valid detail
• preserve the selected scalar field and structured details when replaying assistant history
• retain encrypted reasoning details on matching tool calls for continuation
• accept the known interleaved field names while tolerating provider-specific strings
• add recorded reasoning and signed-continuation cassettes for two compatible gateways

Testing

• bun test test/provider/openai-chat.test.ts test/provider/openai-chat-reasoning.recorded.test.ts (packages/ai)
• bun typecheck (packages/ai)
• bun test test/config/provider-v1.test.ts (packages/core)
• bun typecheck (packages/core)
• live cassette recording followed by offline replay for both gateway fixtures

Fixes #37553

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