fix: not-equals on missing attribute returns false instead of true

[AnatolyRugalev]

Summary

The <> (not-equals) comparison operator returns false when either operand is a missing attribute. DynamoDB returns true — a non-existent value is not equal to any value.

Version: dynoxide 0.9.8

Reproduction

dynoxide --port 8000 &

export AWS_ACCESS_KEY_ID=fake AWS_SECRET_ACCESS_KEY=fake \
       AWS_DEFAULT_REGION=us-east-1 EP=http://localhost:8000

aws dynamodb create-table --endpoint-url $EP \
  --table-name test --billing-mode PAY_PER_REQUEST \
  --attribute-definitions AttributeName=pk,AttributeType=S \
  --key-schema AttributeName=pk,KeyType=HASH

# PutItem with condition: status <> "working" on a non-existent item
# Should succeed — missing attribute is not equal to "working"
aws dynamodb put-item --endpoint-url $EP --table-name test \
  --item '{"pk":{"S":"item1"},"status":{"S":"idle"}}' \
  --condition-expression '#s <> :v' \
  --expression-attribute-names '{"#s":"status"}' \
  --expression-attribute-values '{":v":{"S":"working"}}'
# => ConditionalCheckFailedException (should succeed)

Expected

PutItem succeeds. status <> "working" is true because the item has no status attribute.

Root cause

In condition.rs, the Comparison evaluator returns false for all operators when either operand resolves to None:

match (lv, rv) {
    (Some(l), Some(r)) => Ok(compare_values(&l, op, &r)),
    _ => Ok(false),  // ← should be true for Ne
}

Fix

Return true for <> when either operand is missing, false for all other operators:

_ => Ok(matches!(op, CompOp::Ne)),

Regression tests cover all six comparison operators on missing attributes. Also submitted as a conformance test: nubo-db/dynamodb-conformance#1

[hicksy]

Two small edits pushed to your branch:

1. Rebased onto current main as our workflows have been updated there recently.
2. cargo fmt (minor formatting fixes). No logic change.

As you were a first time contributor the CI checks needed approval. CI is approved and running; I'm leaving benchmark-regression unapproved for now (unrelated housekeeping). Will merge once CI is green.