feat(fusion_accounting_ai): add LLMProvider contract + configurable openai base_url

Phase 1 prerequisite for local LLM support. Adapters now declare
capability flags (supports_tool_calling, max_context_tokens, etc.) so
the engine can reason about what backend is available.

OpenAI adapter accepts fusion_accounting.openai_base_url config -- point
it at LM Studio (http://host.docker.internal:1234/v1) or Ollama
(http://host.docker.internal:11434/v1) and the existing OpenAI adapter
works unchanged.

Implementation note: existing Odoo AbstractModel adapters
(fusion.accounting.adapter.openai/claude) are preserved untouched to
avoid breaking the chat panel; the new plain-Python OpenAIAdapter and
ClaudeAdapter classes (LLMProvider subclasses) are added alongside them.

Made-with: Cursor

fusion_accounting_ai/tests/__init__.py

@@ -1,2 +1,3 @@
 from . import test_post_migration
 from . import test_data_adapters
+from . import test_llm_provider_contract