fix: prompt-injection delimiter escape, legacy config coercion, gather resilience, true cross-thread concurrency bound

Addresses items 4-8 from the max-effort review of PR #272 (VectifyAI/PageIndex#272).

- agent.py: wrap_with_doc_context() strips '<'/'>' from doc_name/doc_description
  (untrusted: unsanitized filename / LLM-generated from document content) before
  inserting them into the <docs>...</docs> block, so embedded content can never
  form a literal </docs> that closes the delimiter early and escapes the
  untrusted-data boundary SCOPED_SYSTEM_PROMPT relies on. Deterministic
  per-field transform, doesn't touch the (cacheable) static system prompt.

- ConfigLoader.load() (legacy 0.2.x compat) now routes merged overrides through
  IndexConfig before returning, so a legacy 'no' string gets pydantic's bool
  coercion instead of surviving as a truthy non-empty string — page_index_main's
  bare `if opt.if_add_node_summary:` checks (changed from `== 'yes'` elsewhere
  in this PR) were silently inverting caller intent and firing unwanted billed
  LLM calls.

- verify_toc, process_large_node_recursively, tree_parser,
  generate_summaries_for_structure, generate_summaries_for_structure_md: added
  return_exceptions=True to their asyncio.gather calls (llm_completion/
  llm_acompletion raise RuntimeError on retry exhaustion, added earlier in this
  PR), each with a degrade path matching the pattern already used by sibling
  hardened gathers in the same files. One transient LLM failure no longer
  aborts the whole document's indexing.

- _llm_semaphore is now a true process-wide ceiling (threading.Semaphore,
  shared across every thread/event loop) instead of one asyncio.Semaphore per
  event loop -- concurrently indexing N documents on N threads no longer
  multiplies the effective cap by N. A max_concurrency_scope() override is
  layered as a second, nested, per-loop restriction that can only tighten the
  effective cap within the ceiling, never widen past it.

- set_llm_params() mutated a bare process-wide dict with no per-call isolation,
  unlike max_concurrency which already had ContextVar scoping. Added
  llm_params_scope() (mirrors max_concurrency_scope) + IndexConfig.llm_params,
  wired into build_index() the same way max_concurrency already was, so
  concurrent indexing jobs with different llm kwargs don't leak into each
  other.

Adds regression tests for all five. Full suite: 221 passed, 2 skipped (one
pre-existing, unrelated flaky cloud-streaming test intermittently fails on
rerun; confirmed independent of this change).

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

tests/test_config.py

@@ -34,3 +34,8 @@ def test_legacy_yes_no_strings_coerce_to_bool():
     config = IndexConfig(if_add_node_id="yes", if_add_node_summary="no")
     assert config.if_add_node_id is True
     assert config.if_add_node_summary is False
+
+
+def test_llm_params_field_defaults_to_none():
+    assert IndexConfig().llm_params is None
+    assert IndexConfig(llm_params={"temperature": 1}).llm_params == {"temperature": 1}