fix(ai): backfill terminal reasoning state

[rekram1-node]

Summary

• decode authoritative reasoning output carried by terminal Responses events
• end reasoning at summary/item completion instead of holding it open until the response terminates
• reconcile later terminal encryption through a metadata-only event and persist that update for continuation
• keep parsing storage-agnostic and validate reasoning item identity by output index
• handle completed, incomplete, done-only, and missing-item-done streams

Testing

• bun test test/provider/openai-responses.test.ts test/provider/xai-responses.test.ts test/provider/google-vertex.test.ts test/provider/openai-compatible-responses.test.ts (packages/ai)
• bun test test/session-runner-tool-events.test.ts (packages/core)
• bun test test/session-runner.test.ts --test-name-pattern "restores durable reasoning provider metadata" (packages/core)
• bun test test/event-manifest.test.ts (packages/schema)
• bun typecheck (packages/ai, packages/core, packages/schema, packages/client)
• bun run generate (packages/client)