feat(core): retain provider finish details - #43332

Summary

• persist raw provider finish reasons and route-specific terminal state on assistant messages
• carry terminal diagnostics through successful and failed durable step events and client projections
• retain content-filter as the assistant finish while preserving the existing provider.content-filter failure behavior
• keep other failed steps as finish: error, even when a provider completed before local tool settlement failed
• regenerate Promise and Effect client contracts

Part of #41932. This PR adds the shared durable plumbing; provider-specific refusal and diagnostic capture remains follow-up work in @opencode-ai/ai.

Testing

• full Core suite: 1,881 tests passed
• focused Schema terminal compatibility tests
• focused Core publisher and Session projection tests
• affected package typechecks
• full monorepo typecheck
• deterministic client regeneration