refactor(index): dedupe the copied indexing pipeline behind deprecation shims

The new SDK copied the legacy indexing pipeline into pageindex/index/
instead of moving it, leaving two divergent copies of page_index.py /
page_index_md.py / utils.py. They had already drifted (the legacy copy
still compared IndexConfig booleans against 'yes' — a separate fix),
and every pipeline change had to be applied twice.

Make pageindex/index/ the single source of truth (same pattern as the
LegacyCloudAPI shim for the 0.2.x cloud SDK):

- pageindex/index/utils.py absorbs the 27 legacy-only helpers/classes
  (get_page_tokens, convert_page_to_int, ConfigLoader, PDF text helpers,
  ...) so it's the sole utils module. Reconciled the diverged funcs:
  kept the modern versions, backported the #331 get_leaf_nodes .get()
  fix, and restored remove_fields' max_len parameter (superset).
- index/page_index*.py now import `from .utils import *`;
  index/legacy_utils.py (a re-export of the old top-level utils) deleted.
- Top-level page_index.py / page_index_md.py / utils.py become thin
  re-export shims that emit PendingDeprecationWarning. The md_to_tree
  shim coerces legacy 'yes'/'no' string flags to bool (the canonical
  version is boolean-typed).
- ConfigLoader no longer reads the deleted config.yaml; it builds
  defaults from IndexConfig (was an unconditional FileNotFoundError).
- __init__.py and retrieve.py import from pageindex.index.* directly so
  `import pageindex` does not trip the shims.

Adds tests/test_legacy_shims.py pinning the contract: clean top-level
import doesn't warn, legacy submodule imports warn, symbols still
resolve, shim and canonical share one implementation, the #331 fix and
ConfigLoader-without-yaml both hold, and the md_to_tree coercion works.

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

pageindex/__init__.py

@@ -1,7 +1,9 @@
 # pageindex/__init__.py
-# Upstream exports (backward compatibility)
-from .page_index import *
-from .page_index_md import md_to_tree
+# Upstream exports (backward compatibility). Import from the canonical
+# pageindex.index.* modules directly so `import pageindex` does NOT trip the
+# top-level deprecation shims (pageindex.page_index / .page_index_md / .utils).
+from .index.page_index import *
+from .index.page_index_md import md_to_tree
 from .retrieve import get_document, get_document_structure, get_page_content
 
 # SDK exports