fix: guard postgres memory upserts by owner

[NeerajCodz]

Motivation

• The PostgreSQL adapter used ON CONFLICT (id) DO UPDATE that unconditionally assigned tenant_id/user_id and other fields from the incoming record, enabling a caller who can supply/collide id to reassign or overwrite another tenant's memory.
• The schema defines id as a global primary key, so the conflict resolution must not allow ownership reassignment during a write.

Description

• Removed updates to tenant_id and user_id from the ON CONFLICT update list in src/adapters/postgres.rs so ownership cannot be overwritten on conflict.
• Added a WHERE nextral_memories.tenant_id = EXCLUDED.tenant_id AND nextral_memories.user_id = EXCLUDED.user_id clause to the ON CONFLICT ... DO UPDATE to ensure conflicted updates only apply when the existing row already belongs to the same tenant and user.
• Capture the number of affected rows from the execute call and return a CoreError::Conflict with a clear message when the INSERT ... ON CONFLICT matched an existing id belonging to a different tenant/user so callers get a deterministic error instead of silent overwrite.

Testing

• Ran cargo test -q and all tests passed (11 passed, 0 failed).
• Ran cargo fmt --check which failed due to unrelated/pre-existing formatting differences outside this patch.

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Codex Task