refactor(agent-core-v2): rename xstate inspection envelope ids to actorId and parentActorId

[sailist]

Related Issue

Internal follow-up to #3687 (no external issue).

Problem

The xstate inspection envelope streamed over /api/v1/debug/ws named the xstate actor's sessionId as actorSessionId, which collides with the existing agent session-id concept inside the codebase and makes the debug stream hard to read. The newly added parent field inherited the same problem as parentSessionId.

What changed

• Renamed the envelope fields in XstateInspectionEnvelope: actorSessionId → actorId (still the xstate actor instance sessionId) and parentSessionId → parentActorId.
• The previous actorId field (the user-facing actorRef.id) is kept under the new name refId to free up the actorId name.
• Updated the two envelope assertions in machine.test.ts and wsUpgradeAuth.test.ts.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue (external PRs: the issue must have a maintainer's /approve).
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update.

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