Complete the #93527 fix: the tombstone now carries the human prompt
text via _entry_prompt_text (handling flat prompt, ShareGPT, and
chat-style shapes), _scan_completed_prompts_by_content counts
discarded rows as completed instead of only reading ShareGPT
conversations, and the merge step reports excluded tombstones in the
combined-count summary. Adds a dedicated regression suite covering the
tombstone round-trip, the all-discarded-batch resume path, and merge
exclusion.
Salvaged from #93579 (issue reporter's PR), building on #93542.
Fixes#93527
aniruddhaadak80