feat(closes OPEN-11315): modernize the LangChain/LangGraph callback handler for v1

Modernizes the sync + async Openlayer LangChain/LangGraph callback handler for
LangChain v1 (OPEN-11315). The callbacks API stayed backwards-compatible across
v1, so these are gap-fills and modernizations rather than a migration.

High-priority fixes:
- Wire on_retriever_start/end/error into the sync handler (previously only the
  async handler had them) so synchronous RAG pipelines produce a RETRIEVER step
  and populate the context column; fall back to the handler-owned trace when a
  root-level retriever has no external trace context.
- Fall back to serializing AIMessage.tool_calls in _extract_output when the
  generation text is empty, and preserve tool_calls in _message_to_dict, so
  tool-only agent turns no longer record empty output.

Modernization:
- Tokens: read AIMessage.usage_metadata first, then llm_output / generation_info;
  capture input/output token details (cache_read, cache_creation, reasoning)
  under step metadata.
- Provider: use metadata["ls_provider"] as the primary source (mapped to
  Openlayer provider names), with the _type map and LiteLLM prefixes as fallbacks.
- LangGraph metadata: chain steps prefer the runnable's own name, falling back to
  langgraph_node, then the serialized id (matching the TS handler) so node
  internals keep their real names; metadata["thread_id"] auto-maps to session_id
  unless an explicit session is set (opt-out via map_thread_id_to_session).
- v1 content blocks: normalize list / content-block message content (text joined
  into content, non-text blocks preserved under content_blocks).
- Drop the removed langchain.schema / langchain.callbacks.base import fallback;
  import from langchain_core only.

Tests:
- Add an offline test suite for the handler (real langchain_core objects,
  publishing disabled).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: viniciusdsmello