fix(sdk): wake embedded session live tail

[kitlangton]

Summary

• reuse the reviewed unified Session stream so execution activity wakes embedded subscribers even when location-scoped EventV2 commits use another service instance
• drain durable SQLite rows before emitting inactive activity, preserving aggregate order without polling
• add an sdk-next embedded regression that subscribes before execution, runs tool turn A, fails provider turn B, and observes every durable row through session.next.step.failed on the still-open stream

This is the current-dev integration of the reviewed work from #34008. It has no dependency on continuation defaults, provider-error typing, or #34010.

Verification

• packages/sdk-next: bun run test (4 passed)
• packages/core: bun run test test/event.test.ts test/session-prompt.test.ts test/session-run-coordinator.test.ts (90 passed)
• packages/client: bun run check:generated && bun run test (9 passed)
• legacy JS SDK regeneration (packages/sdk/js/script/build.ts) produced no additional diff
• bun run typecheck (29 packages passed)
• bun run lint (0 errors; existing warnings only)
• bunx prettier --check $(git diff --name-only origin/dev)
• git diff --check

Full Core suite: 1031 passed; one unrelated macOS watcher assertion failed because .git/HEAD was reported as add instead of expected change (test/filesystem/watcher.test.ts). The same isolated watcher test reproduces the failure.