feat(ai): opt-in LangGraph product assistant backend

[gianlucamazza]

Summary

Opt-in LangGraph product path behind the existing AssistantRuntime facade. Default remains chat (chat_agent_tool_loop). No new public CLI commands.

• assistant_backend setting / ASSISTANT_BACKEND=chat|langgraph
• GraphAssistantBackend: model↔tools StateGraph, ReAct when provider lacks native tools, plain generate otherwise
• Shared prompt helpers (ai.runtime.prompt) used by chat + langgraph
• Node-granularity StreamEvent streaming on the graph path
• Status exposes assistant_backend + assistant_backend_id
• Populates context.available_tools so native tool loops actually engage
• build_assistant_graph never re-enters runtime.run
• Typed contracts (BaseLLMProvider, Protocol); no cast / type: ignore / noqa

Non-goals (this PR)

• Default flip to langgraph (later, after soak)
• Multi-agent workflows on CLI
• Token-level streaming inside graph model node (v1 is node-granularity)

Test plan

• ruff check + mypy openfatture/ai/runtime
• Parity suite: tests/ai/test_assistant_backend_parity.py
• Runtime / tool-loop / status readiness tests
• CI green on this PR
• Manual (optional): ASSISTANT_BACKEND=langgraph openfatture assistant "…"

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📥 Commits

Reviewing files that changed from the base of the PR and between a99e113 and 8f55daa.

📒 Files selected for processing (16)

• CHANGELOG.md
• docs/AI_ARCHITECTURE.md
• docs/CONFIGURATION.md
• docs/STATUS.md
• openfatture/ai/agents/chat_agent.py
• openfatture/ai/runtime/__init__.py
• openfatture/ai/runtime/constants.py
• openfatture/ai/runtime/graph.py
• openfatture/ai/runtime/graph_backend.py
• openfatture/ai/runtime/prompt.py
• openfatture/ai/runtime/service.py
• openfatture/cli/commands/status.py
• openfatture/platform/config.py
• tests/ai/test_assistant_backend_parity.py
• tests/ai/test_assistant_runtime.py
• tests/ai/test_tool_loop_parity.py

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[github-actions]

Performance Test Results

Benchmark results file is empty (no benchmarks collected)

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View detailed results in the workflow artifacts