Read document bytes from an arbitrary source bucket 🟥

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

infrastructure/s3/s3_document_bytes_reader.py

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+"""Reads document bytes from arbitrary S3 buckets (ADR-0060).
+
+Uploaded files live across many source buckets (each `uploaded_files` row
+carries its own `s3_file_bucket`), so — unlike the bucket-bound `S3Client` —
+this reader takes the bucket per call. Used by the Bulk Document Download
+orchestrator to stream each chosen file into the ZIP.
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+
+class S3DocumentBytesReader:
+    def __init__(self, boto_s3_client: Any) -> None:
+        self._client = boto_s3_client
+
+    def read(self, bucket: str, key: str) -> bytes:
+        raise NotImplementedError

tests/infrastructure/test_s3_document_bytes_reader.py

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+"""The multi-bucket document reader (ADR-0060) — reads object bytes from
+whichever source bucket an uploaded file lives in."""
+
+from collections.abc import Iterator
+
+import pytest
+from moto import mock_aws
+
+from infrastructure.s3.s3_document_bytes_reader import S3DocumentBytesReader
+from tests.infrastructure import make_boto_client
+
+
+@pytest.fixture
+def reader() -> Iterator[S3DocumentBytesReader]:
+    with mock_aws():
+        boto_client = make_boto_client("s3")
+        boto_client.create_bucket(Bucket="src-a")
+        boto_client.create_bucket(Bucket="src-b")
+        boto_client.put_object(Bucket="src-a", Key="surveys/one.pdf", Body=b"AAA")
+        boto_client.put_object(Bucket="src-b", Key="photos/two.zip", Body=b"BBB")
+        yield S3DocumentBytesReader(boto_client)
+
+
+def test_reads_bytes_from_the_named_source_bucket(
+    reader: S3DocumentBytesReader,
+) -> None:
+    # act / assert — each object is read from its own bucket.
+    assert reader.read("src-a", "surveys/one.pdf") == b"AAA"
+    assert reader.read("src-b", "photos/two.zip") == b"BBB"