fix: six P2 correctness/robustness cleanups from the SDK review

- legacy call_llm: open AsyncOpenAI via `async with` so the client (and
  its HTTP connection pool) is closed instead of leaked.
- LocalBackend.add_document: fail fast with CollectionNotFoundError when
  the collection doesn't exist, before the expensive parse + LLM index
  (previously the missing FK only tripped at save time, after paying for
  the LLM work). Also raise builtin FileNotFoundError for a missing path
  instead of FileTypeError (which now means only "unsupported extension").
- Collection.query(doc_ids=None): the empty-collection guard now always
  runs — previously it was skipped once PAGEINDEX_EXPERIMENTAL_MULTIDOC
  was set. A single list_documents call serves both the guard and the
  multi-doc warning (no separate call just to decide whether to warn).
- CloudBackend.query_stream: on early consumer break / GeneratorExit,
  signal the background SSE thread to stop and force-close the response,
  so it no longer drains the whole stream in the background.

Adds regression tests for each (client closed, fail-fast on unknown
collection, FileNotFoundError, empty-check under the multidoc env flag,
single list call, early-break thread stop).

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

pageindex/index/utils.py

@@ -460,12 +460,12 @@ async def call_llm(prompt, api_key, model="gpt-4.1", temperature=0):
     """
     import openai
 
-    client = openai.AsyncOpenAI(api_key=api_key)
-    response = await client.chat.completions.create(
-        model=model,
-        messages=[{"role": "user", "content": prompt}],
-        temperature=temperature,
-    )
+    async with openai.AsyncOpenAI(api_key=api_key) as client:
+        response = await client.chat.completions.create(
+            model=model,
+            messages=[{"role": "user", "content": prompt}],
+            temperature=temperature,
+        )
     return response.choices[0].message.content.strip()