fix(ai): preserve response message phases

[rekram1-node]

Summary

• align Open Responses assistant phase handling with the official OpenAI SDK contract (commentary, final_answer, and explicit null)
• preserve phase metadata across streamed output ordering variants and replay it on follow-up requests
• persist assistant text provider state through V2 durable events, projections, history reconstruction, and generated client types
• retain phase across same-provider model changes and partial failures without leaking metadata across providers

Verification

• packages/ai: bun test test/provider/openai-responses.test.ts; bun typecheck
• packages/core: bun test test/session-runner.test.ts; bun test test/session-runner-message.test.ts; bun typecheck
• packages/schema: bun typecheck; bun test test/event-manifest.test.ts
• packages/client: bun run generate
• iterative scoped sub-agent review completed with no findings

Notes

• Full packages/schema tests have one pre-existing unrelated failure in contract-hygiene.test.ts for agent color validation; it is outside this PR scope.