Every remaining ThreadMetaStore method gains a `workspace_id` keyword
mirroring `user_id`:
- `search()` adds WHERE workspace_id and (for the memory impl) folds it
into the BaseStore filter dict.
- `update_display_name` / `update_status` / `update_metadata` / `delete`
no-op if the row lives in a different workspace. The SQL helper
`_check_ownership()` was widened to do both checks in one pass.
- `MemoryThreadMetaStore._get_owned_record()` likewise takes both ids.
5 new isolation tests prove writes from workspace B against workspace A's
thread are silently dropped (no row mutation when the caller re-reads
from workspace A). 38 existing thread_meta / owner / memory-store tests
still pass.
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