feat(app-server): move v2 sessionId onto Thread

[jif-oai]

Why

session_id and thread_id are separate identities after #20437, but app-server only surfaced sessionId on the thread/start, thread/resume, and thread/fork response envelopes. Other thread-bearing surfaces such as thread/list, thread/read, thread/started, thread/rollback, thread/metadata/update, and thread/unarchive either lacked the grouping key or forced clients to special-case those three responses.

Making sessionId part of the reusable Thread payload gives every v2 API surface one place to expose session-tree identity.

Mental model

1. thread.sessionId lives on Thread
2. It is a view/runtime identity for the current live session tree, not durable stored lineage metadata
3. When app-server has a live loaded thread, it copies the real value from core’s session_configured.session_id
4. When it only has stored/unloaded data, it falls back to thread.sessionId = thread.id

What changed

• Added sessionId to the v2 Thread.
• Removed the duplicate top-level sessionId fields from thread/start, thread/resume, and thread/fork; clients should now read response.thread.sessionId.
• Populated thread.sessionId when building live thread responses, replaying loaded threads, and returning stored-thread summaries so the field is present across start, resume, fork, list, read, rollback, metadata-update, unarchive, and thread/started paths. See load_thread_from_resume_source_or_send_internal and thread_from_stored_thread.
• Preserved the stored-thread fallback: if a thread has not been loaded into a live session tree yet, thread.sessionId falls back to thread.id; once the thread is live again, the field reports the active session tree root.
• Regenerated the JSON/TypeScript schemas and updated the app-server README examples to show thread.sessionId on the thread object.

[jif-oai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Another round soon, please!

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