memory_purge creates safety snapshots even when zero memories are deleted

[loveRhythm1990]

Summary

memory_purge creates a safety snapshot even when no memories are actually deleted. This happens for no-op purge requests (e.g. nonexistent session or topic), causing unnecessary snapshot accumulation.

Steps to Reproduce

1. Purge by a nonexistent session:

   {
     "session_id": "nonexistent-session-xyz",
     "reason": "test"
   }

   Response:

   Purged 0 memory(s) for session 'nonexistent-session-xyz'
   Safety snapshot: mem_snap_mem_u_d33c_27b1de26df_pre_purge_362890af
   

2. Purge by a non-matching topic:

   {
     "topic": "FUNC-TEST-NO-MATCH-XYZ",
     "reason": "test"
   }

   Response:

   Purged 0 memory(s) matching 'FUNC-TEST-NO-MATCH-XYZ'
   Safety snapshot: mem_snap_mem_u_d33c_27b1de26df_pre_purge_9635fe31
   

Expected Behavior

When 0 memories are purged, no safety snapshot should be created.

Alternatively, provide a dry_run mode so callers can validate purge criteria without side effects.

Actual Behavior

Even zero-result purge operations generate pre_purge_* safety snapshots.

During testing, the snapshot count increased from 1 to 3 after two no-op purge calls.

Impact

• Snapshot list pollution over time
• Harder snapshot management for operators
• Unexpected side effects for exploratory/debug purge calls

Environment

• MCP server: http://localhost/memoria-api/mcp
• Tool: memory_purge
• Discovered during functional testing on 2026-06-25

Suggested Fix

Skip safety snapshot creation when the purge result count is 0, or gate snapshot creation behind an explicit opt-in flag.