chore: import upstream snapshot with attribution

test/fuzz/README.md

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+# Fuzz targets
+
+[Atheris](https://github.com/google/atheris) fuzz harnesses for Haystack's
+untrusted-input entry points. They are wired into CI via
+[ClusterFuzzLite](https://google.github.io/clusterfuzzlite/)
+(see [`.clusterfuzzlite/`](../../.clusterfuzzlite) and the
+`ClusterFuzzLite PR fuzzing` workflow).
+
+| Harness | Target | Why |
+|---|---|---|
+| `fuzz_pipeline_loads.py` | `Pipeline.loads` | Deserializing a serialized pipeline (YAML) is a documented attack surface. |
+| `fuzz_document_from_dict.py` | `Document.from_dict` | Reconstructing a `Document` from an untrusted dict. |
+| `fuzz_filters.py` | `document_matches_filter` | Evaluating an untrusted filter expression. |
+
+Each harness catches the exceptions that are a *normal* reaction to malformed
+input; anything else (a crash, unbounded recursion, a hang, or an unexpected
+exception type) is reported by Atheris as a finding. The "expected" exception
+lists can be tightened over time to surface more subtle bugs.
+
+## Seed corpus
+
+`fuzz_document_from_dict` and `fuzz_filters` parse the raw fuzzer input as JSON,
+so the input domain *is* JSON text. A small seed corpus of valid inputs lives in
+[`corpus/<harness>/`](corpus) to bootstrap coverage past the JSON parse — without
+it a short run spends most of its budget producing inputs that don't parse. The
+ClusterFuzzLite build (`.clusterfuzzlite/build.sh`) zips each `corpus/<harness>/`
+into the `<harness>_seed_corpus.zip` the runner expects. Add a new valid input by
+dropping a `.json` file into the matching directory.
+
+## Run locally
+
+```sh
+pip install atheris
+pip install -e .
+
+# Fuzz for a bit (Ctrl-C to stop); -atheris_runs limits the number of inputs.
+# Pass the seed corpus dir so libFuzzer starts from valid inputs.
+python test/fuzz/fuzz_pipeline_loads.py -atheris_runs=100000
+python test/fuzz/fuzz_document_from_dict.py test/fuzz/corpus/fuzz_document_from_dict -atheris_runs=100000
+python test/fuzz/fuzz_filters.py test/fuzz/corpus/fuzz_filters -atheris_runs=100000
+```
+
+A directory argument is used as a seed corpus (and grown with new coverage); a
+crashing input file reproduces a finding:
+
+```sh
+python test/fuzz/fuzz_filters.py crash-<hash>       # reproduce a crash
+```
+
+> Note: Atheris builds a native extension and is not part of the dev
+> dependencies; install it on demand as shown above. `pytest` does not collect
+> these files (they are named `fuzz_*.py`, not `test_*.py`).