Web Shell history boundary pagination

Problem

Web Shell starts restored-history pagination with beforeRecordId, then switches to the opaque nextCursor returned by the transcript endpoint. A cursor can be rejected with invalid_transcript_cursor when the next request is handled by a runtime that cannot validate the original HMAC. The user then cannot load the remaining history.

Design

Continue backward Web Shell pagination with record boundaries whenever a page contains a persisted record id. Replay already stamps qwen.session.recordId on transcript events, and beforeRecordId is exclusive, so the earliest record id in the returned page is the next boundary.

Keep nextCursor as a compatibility fallback for older daemons or malformed pages that do not expose any persisted record id. No daemon API or cursor security semantics change.

Verification

• A multi-page transcript uses beforeRecordId for every Web Shell request.
• Events already displayed remain deduplicated.
• A page without a persisted record id still advances with nextCursor.
• Existing retry, terminal-error, capacity, and malformed-event behavior stays unchanged.