add unit test coverage for QaRankingCodegen

### Task Summary

Add a dedicated `QaRankingCodegenSpec.scala` that pins the payload/parse Python-snippet contract of `QaRankingCodegen` — the Hugging Face codegen for the question-answering / ranking task family. Mirror the existing `TextGenCodegenSpec` (same package) as the template.

## Background

`QaRankingCodegen` (`common/workflow-operator/src/main/scala/org/apache/texera/amber/operator/huggingFace/codegen/QaRankingCodegen.scala`) is an `object extends TaskCodegen`. It produces two Python source snippets — `payloadPython(ctx)` and `parsePython(ctx)` — that are spliced into the generated inference operator. It handles five HF pipeline tasks and, like every codegen, must reference `self.*` attributes rather than inlining raw user strings from the `CodegenContext`.

```scala
override val task: String = "question-answering"
override val tasks: Set[String] = Set(
"question-answering", "table-question-answering",
"zero-shot-classification", "sentence-similarity", "text-ranking"
)
```

## Behavior to pin

| Member | Contract |
| --- | --- |
| `task` | equals `"question-answering"` |
| `tasks` | equals exactly the 5-element set above |
| `payloadPython` | opens with `if task == "question-answering":`; has an `elif` branch for table-QA, zero-shot, sentence-similarity and text-ranking; ends with the `else: payload = {"inputs": prompt_value}` fallback |
| `parsePython` | the QA / table-QA branches read `body.get("answer", ...)`; zero-shot / sentence-similarity / text-ranking return `json.dumps(body)` |
| no raw-value leakage | with sentinel `CodegenContext` string fields (e.g. `contextColumn` / `candidateLabels` / `sentencesColumn` set to distinctive `"MARKER_…"` values), neither snippet contains the sentinel — the code references `self.CONTEXT_COLUMN` / `self.CANDIDATE_LABELS` / `self.SENTENCES_COLUMN` instead |

Build the `CodegenContext` with a small `makeCtx` helper exactly like `TextGenCodegenSpec` (set `task = "question-answering"` plus the QA-specific columns). Assert on snippet structure / marker substrings, not on exact whitespace.

## Scope

- New spec: `QaRankingCodegenSpec.scala` under `common/workflow-operator/src/test/scala/org/apache/texera/amber/operator/huggingFace/codegen/`.
- No production-code changes.

### Task Type

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

Read common/workflow-operator/src/main/scala/org/apache/texera/amber/operator/huggingFace/codegen/QaRankingCodegen.scala and mirror common/workflow-operator/src/test/scala/org/apache/texera/amber/operator/huggingFace/codegen/TextGenCodegenSpec.scala. Add QaRankingCodegenSpec.scala with makeCtx and structure-based assertions for task, tasks, payloadPython, parsePython, and sentinel non-leakage; done means the new spec passes without production changes.

Written by the indexing model from the issue text.