fix(events): refuse an empty session id

[justintime4tea]

Summary

SessionId accepted the empty string, and every store keyed by session inherited it (#800): clients sending a blank chat session id shared one skill log, and a session journal (#779) keyed by a blank would be shared by every run whose id resolved to one. This moves SessionId onto nonempty_string_newtype! so a blank cannot be held at all, and has each boundary that built one from a string treat a blank as "no session".

"Stacked" on #794 — draft until it lands

This builds on nonempty_string_newtype! from #794, so its base is #794's branch. The typed resolver builds on #795 (treat a blank chat session id as missing), which has merged into nightly; the branch is rebased onto #794 at the current nightly and no longer carries its own copy.

When #794 merges: retarget to nightly, mark ready.

Changes

• aura-events: SessionId is generated by nonempty_string_newtype! — new returns Result<Self, EmptyId>, TryFrom replaces From, "" fails to deserialize. The macro gains non_empty(value) -> Option<Self> for boundaries where blank and absent mean the same.
• chat handler: chat_session_id() returns a SessionId (blank sources still fall through, a fresh id is minted if none), and prepare_request takes &SessionId, so the guarantee travels with the value.
• HITL scopes built by the builder and the orchestrator, and ScopeRecord → AgentScope on restoring a stored approval: a blank reads as None rather than a session or a decode failure.
• standalone CLI (direct.rs): a blank session id is an error at the boundary; the Backend trait keeps its &str.
• tests: refusal on construction and serde, non_empty, a blank in a stored scope restoring as None, and the existing resolver and skill-log tests under the typed id.

[greptile-apps]

Confidence Score: 5/5

[Medium impact] The PR appears safe to merge.

Summary

SessionId now rejects empty strings during construction and deserialization.

• Chat requests carry the typed identifier into prepare_request.
• Empty optional session strings become None in approval scopes.
• Standalone CLI requests reject an empty identifier.
• Tests cover construction, deserialization, and stored scope restoration.

No new actionable issues were found. The earlier header-fixture finding was withdrawn, and the earlier comment changes are addressed.

_{Reviews (4) · Last reviewed commit: "fix(events): refuse an empty session id" · Reviewed by Greptile}