fix(postgres): prevent cross-tenant memory overwrite in upsert

[NeerajCodz]

Motivation

• The upsert_memory path previously used ON CONFLICT (id) DO UPDATE and unconditionally reassigned tenant_id/user_id and other fields, allowing a caller-supplied or guessed global id to overwrite another tenant's memory record.
• The schema uses a global id primary key, so ownership must be enforced at write time to prevent cross-tenant corruption.

Description

• Updated src/adapters/postgres.rs upsert_memory to stop updating tenant_id/user_id on conflict and to add a WHERE nextral_memories.tenant_id = EXCLUDED.tenant_id AND nextral_memories.user_id = EXCLUDED.user_id guard to the ON CONFLICT ... DO UPDATE clause.
• Capture the rows_affected returned from execute and return a CoreError::Conflict when no rows were updated, which indicates an id collision with a different tenant/user scope.
• Preserves existing behavior for same-tenant updates and otherwise fails the write rather than silently moving ownership.

Testing

• Ran the unit test suite with cargo test -p nextral --lib and all tests passed (11 passed, 0 failed).

---

Codex Task