fix: exact md page selection + restore CHATGPT_API_KEY alias

Address PR #272 review:

- get_md_page_content / retrieve._get_md_page_content returned every node
  whose line_num fell in [min(pages), max(pages)], so a non-contiguous spec
  like "5,100" over-fetched everything in between. Match the exact requested
  line numbers instead, mirroring the PDF path. (Same bug as #280.)
- Restore the CHATGPT_API_KEY -> OPENAI_API_KEY backward-compat alias dropped
  when pageindex/utils.py became a re-export shim; users with only
  CHATGPT_API_KEY set would otherwise fail auth after upgrading. It now runs
  in __init__.py right after load_dotenv.

Adds regression tests for both.

Claude-Session: https://claude.ai/code/session_01Kx5DgKbhK1N8autqXH8SmS

tests/test_page_content.py

@@ -0,0 +1,36 @@
+"""Markdown page-content selection must return exactly the requested lines,
+mirroring the PDF path — not the whole [min, max] range (PR #272 review / #280)."""
+
+
+def _md_structure():
+    # line_num 40 sits *between* 5 and 100 but is NOT requested below.
+    return [
+        {"line_num": 5, "text": "line five", "nodes": [
+            {"line_num": 40, "text": "line forty (should be excluded)", "nodes": []},
+        ]},
+        {"line_num": 100, "text": "line hundred", "nodes": []},
+        {"line_num": 101, "text": "line 101", "nodes": []},
+    ]
+
+
+def test_get_md_page_content_returns_only_requested_lines():
+    from pageindex.index.utils import get_md_page_content
+
+    out = get_md_page_content(_md_structure(), [5, 100])
+    # exactly the two requested lines — not 5, 40, 100 (the old range behavior)
+    assert [r["page"] for r in out] == [5, 100]
+    assert all("forty" not in r["content"] for r in out)
+
+
+def test_get_md_page_content_empty_spec():
+    from pageindex.index.utils import get_md_page_content
+
+    assert get_md_page_content(_md_structure(), []) == []
+
+
+def test_retrieve_md_page_content_returns_only_requested_lines():
+    # The legacy retrieve path has its own copy of the same logic.
+    from pageindex.retrieve import _get_md_page_content
+
+    out = _get_md_page_content({"structure": _md_structure()}, [5, 100])
+    assert [r["page"] for r in out] == [5, 100]