add unit test coverage for LogicalLink

[aglinxinyuan]

amber/src/main/scala/org/apache/texera/workflow/LogicalLink.scala is the case class describing an edge in the user-facing logical workflow graph. It has two construction paths and ships across the wire / persisted in saved workflows, but it has no unit test today:

Surface	Need to pin
Primary constructor	(OperatorIdentity, PortIdentity, OperatorIdentity, PortIdentity) round-trips into the four fields.
Secondary @JsonCreator constructor	(String, PortIdentity, String, PortIdentity) wraps each string in OperatorIdentity(...) — this is the Jackson deserialization path, used to read user-saved workflow JSON.
Case-class equals / hashCode	Structural equality across the four fields.
Jackson deserialization (single-direction)	objectMapper.treeToValue from a JSON object with raw-string fromOpId / toOpId (the shape user-saved workflows actually have) deserializes correctly via the @JsonCreator String overload. Note: full writeValueAsString → readValue round-trip is currently not supported — writeValueAsString emits OperatorIdentity as {"id":"op-A"} (object form), but the @JsonCreator String overload Jackson dispatches to on read can't accept an object. The spec characterizes this asymmetry rather than asserting a passing round-trip; a follow-up fix (additional @JsonCreator object overload or a custom @JsonDeserialize) would flip the characterization-pin into a real round-trip.
Field-name pins	fromOpId / toOpId are pinned by @JsonProperty annotations; fromPortId / toPortId derive from Scala parameter names (no annotation). Both are pinned so a Scala-side rename can't silently break saved workflows.
Missing-field default behavior	Pin the current "no validation in the @JsonCreator path" — missing op-id fields silently produce OperatorIdentity(null) instead of throwing; a future hardening should fail this on purpose.

Priority

P3 - Low

Task Type

• Testing / QA