add unit test coverage for operator-package utilities (OperatorDescriptorUtils + PropertyNameConstants + PortDescriptor)

### Task Summary

Add dedicated unit-specs for three small operator-package utility / contract classes in `common/workflow-operator/`. None of them carry heavy infrastructure dependencies; the value is regression coverage against accidental renames, default-value drift, and Jackson-annotation drift.

## Background

| Source class | Package | Purpose |
| --- | --- | --- |
| `OperatorDescriptorUtils` | `operator.util` | Holds `equallyPartitionGoal(goal, totalNumWorkers)` and `toImmutableMap(javaMap)` — utility pure functions used by worker / partition planning |
| `PropertyNameConstants` | `operator.metadata` | Holds the canonical JSON-property-name constants used across the engine (e.g. `OPERATOR_ID`, `INPUT_PORTS`, `WORKFLOW_ID`). Drift in any constant breaks every persisted workflow JSON that uses it. |
| `PortDescriptor` (+ `PortDescription`) | `operator` | `PortDescriptor` is a trait with mutable `inputPorts` / `outputPorts` lists; `PortDescription` is a case class with the Jackson-annotated `@JsonIgnoreProperties("allowMultiInputs")` backward-compat shim |

## Behavior to pin

### `OperatorDescriptorUtils.equallyPartitionGoal`

| Surface | Contract |
| --- | --- |
| Exactly divisible (`goal % totalNumWorkers == 0`) | every worker gets `goal / totalNumWorkers` |
| Inexact (`goal % totalNumWorkers > 0`) | first `goal % totalNumWorkers` workers get an extra `1`; the rest get the floor |
| Result length | always equals `totalNumWorkers` |
| Sum of the result | always equals `goal` |
| `goal = 0` | all workers get 0 |
| Single-worker | one entry equal to `goal` |

### `OperatorDescriptorUtils.toImmutableMap`

| Surface | Contract |
| --- | --- |
| Empty java map | returns the empty Scala map |
| Populated java map | returns a Scala immutable map with the same entries |
| Subsequent mutation of the source java map | does NOT leak into the returned Scala map |

### `PropertyNameConstants`

| Surface | Contract |
| --- | --- |
| Logical-plan keys (`OPERATOR_ID` / `OPERATOR_TYPE` / `OPERATOR_LIST` / `OPERATOR_LINK_LIST` / `OPERATOR_VERSION` / `ORIGIN_OPERATOR_ID` / `DESTINATION_OPERATOR_ID`) | exact string values |
| Common operator keys (`ATTRIBUTE_NAMES` / `ATTRIBUTE_NAME` / `RESULT_ATTRIBUTE_NAME` / `SPAN_LIST_NAME` / `TABLE_NAME`) | exact string values |
| Physical-plan keys (`WORKFLOW_ID` / `EXECUTION_ID` / `PARALLELIZABLE` / `LOCATION_PREFERENCE` / `PARTITION_REQUIREMENT` / `INPUT_PORTS` / `OUTPUT_PORTS` / `IS_ONE_TO_MANY_OP` / `SUGGESTED_WORKER_NUM`) | exact string values |
| All constants together | distinct (no accidental aliasing) |

### `PortDescriptor` / `PortDescription`

| Surface | Contract |
| --- | --- |
| `PortDescriptor` (mixed into a test class) | defaults `inputPorts` / `outputPorts` to `null` (Jackson sets them post-construction); both are mutable `var`s |
| `PortDescription()` constructor | preserves every field |
| `PortDescription` equality / `copy` | case-class semantics |
| `@JsonIgnoreProperties("allowMultiInputs")` | present on `PortDescription` — verified via reflection (backward-compat shim) |
| JSON round-trip | preserves every field |

## Scope

- New spec files (one per source class per the spec-filename convention):
- `OperatorDescriptorUtilsSpec.scala`
- `PropertyNameConstantsSpec.scala`
- `PortDescriptorSpec.scala`
- No production-code changes.

### Task Type

- [ ] Refactor / Cleanup
- [ ] DevOps / Deployment / CI
- [x] Testing / QA
- [ ] Documentation
- [ ] Performance
- [ ] Other