fix: address PR #272 review findings (directly-fixable items)

Verified against current dev; the compat/behavior decisions (#7 api_key
semantics, #10 CLI flags, #11 doc-description default) are deferred.

Crashes:
- page_index(): snapshot args before importing IndexConfig — locals() was
  capturing the imported class and IndexConfig(extra='forbid') made every call
  raise ValidationError.
- process_none_page_numbers: pop('page', None) instead of del (a TOC item
  without 'page' raised KeyError mid-pipeline).
- pipeline._run_async: guard only the loop detection, not the run, so a real
  RuntimeError from the coroutine isn't masked as "asyncio.run() cannot be
  called from a running event loop".

Silent-wrong / robustness:
- LocalBackend.get_document_structure and the agent get_document /
  get_document_structure tools now surface a missing doc (raise / error-JSON)
  instead of returning empty, matching get_page_content and the cloud backend.
- cloud delete_collection drops the cached folder_id.
- cloud query raises on an empty collection instead of POSTing doc_id:[].
- LocalClient skips the API-key check for keyless providers (ollama, lm_studio,
  …) so keyless LiteLLM models aren't rejected at construction.

Compat / cleanup:
- md_to_tree coerces legacy 'yes'/'no' string flags (a bare 'no' was truthy).
- FileTypeError also subclasses ValueError (0.2.x raised ValueError).
- _validate_llm_provider no longer mutates global litellm.model_cost_map_url.
- __all__ re-includes legacy exports (page_index, md_to_tree, get_*).
- Rewrite examples/agentic_vectorless_rag_demo.py to the Collection API and use
  the in-repo attention.pdf (the old workspace=/client.index/client.documents
  API no longer exists).

Adds tests/test_review_fixes.py (10 regressions). Full suite: 189 passed.

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

pageindex/client.py

@@ -113,14 +113,20 @@ class PageIndexClient:
 
     @staticmethod
     def _validate_llm_provider(model: str) -> None:
-        """Validate model and check API key via litellm. Warns if key seems missing."""
+        """Validate the model string and require an API key for providers that
+        need one. Local / keyless providers (ollama, lm_studio, …) are skipped so
+        a keyless LiteLLM model isn't rejected at construction time."""
         try:
             import litellm
-            litellm.model_cost_map_url = ""
             _, provider, _, _ = litellm.get_llm_provider(model=model)
         except Exception:
             return
 
+        # LiteLLM providers that run locally and need no API key.
+        keyless = {"ollama", "ollama_chat", "lm_studio", "hosted_vllm", "vllm"}
+        if provider in keyless:
+            return
+
         key = litellm.get_api_key(llm_provider=provider, dynamic_api_key=None)
         if not key:
             import os