fix(core): settle interrupted assistant steps

[kitlangton]

Summary

• mark active V2 assistant steps failed when their provider turn is interrupted
• centralize V2 assistant failure publication while preserving raw provider failure behavior
• omit empty projected assistant artifacts from V2 model history without dropping partial output, tools, or provider metadata

This intentionally changes only the V2 Session core.

Testing

• cd packages/core && bun run test -- test/session-runner-message.test.ts test/session-runner.test.ts
• cd packages/core && bun typecheck
• file-scoped Prettier and oxlint (zero errors)
• workspace push-hook typecheck: 23 packages passed