Treat unreadable TUI session JSON as non-empty so GC cannot drop it

[cursor]

session_json_is_empty treated a json_parse throw (and a non-object blob) as an empty transcript. Untitled conversations older than 24h were then hard-deleted on store open, and create_or_reuse reused the row and overwrote the stored session.

Fail closed: only an empty string or a JSON object with empty index / agents counts as empty. Truncated or non-object session JSON is kept. Canonical empty objects still reuse; 24h empty Untitled rows still GC.

unit_conversation_store 27/27 locally (+ ASan, 120 assertions). arbiter compiles.

merge-tree vs main: CLEAN. vs #321–#349: CLEAN (including CHANGELOG this run). Isolated to conversation_store.cpp / its existing test file.

  

---

Note

Low Risk
Localized change to empty-session detection in ConversationStore with regression tests; reduces risk of accidental data loss rather than introducing new behavior paths.

Overview
Unreadable TUI session blobs are no longer treated as empty transcripts. session_json_is_empty now fails closed: JSON parse errors and non-object shapes return non-empty, so only a blank string or a v2 object with empty index / agents counts as empty.

That stops 24h Untitled GC from hard-deleting conversations whose session JSON is truncated or corrupt, and stops create_or_reuse from reusing the active row and overwriting the stored blob. Canonical empty sessions still reuse; truly empty stale Untitled rows still GC.

CHANGELOG updated; unit_conversation_store adds coverage for corrupt JSON, array-shaped sessions, and GC behavior.

^{Reviewed by Cursor Bugbot for commit 918c0d4. Bugbot is set up for automated code reviews on this repo. Configure here.}