test(vector): pin empty-chunk-list parity for all-blank pages

Address claude-review round 5 on PR #868: add
test_all_blank_pages_returns_empty_list documenting that PageAwareChunker
returns [] when every page is blank — and asserting parity with
DocumentChunker, which already returns [] for whitespace-only non-empty
content. The empty-chunk-list case is therefore pre-existing pipeline
behavior, not new to this PR.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/unit/test_document_chunker.py

@@ -439,6 +439,22 @@ class TestPageAwareChunker:
         ]
         assert all(c.page_number is None for c in pa_chunks)
 
+    async def test_all_blank_pages_returns_empty_list(self):
+        """Non-empty content whose every page is blank yields no chunks.
+
+        Matches DocumentChunker, which also returns [] for whitespace-only
+        non-empty content — the downstream pipeline handles an empty chunk
+        list identically for both chunkers.
+        """
+        pages = ["   ", "\n\n", "\t"]
+        content, boundaries = _make_doc(pages)
+
+        chunks = await PageAwareChunker().chunk_text(content, boundaries)
+
+        assert chunks == []
+        # Parity: the char-based chunker behaves the same for blank content.
+        assert await DocumentChunker().chunk_text(content) == []
+
     async def test_empty_content_returns_single_empty_chunk(self):
         """Empty content returns one empty chunk regardless of boundaries."""
         chunks = await PageAwareChunker().chunk_text(