Render structured tool-call arguments in append-only observer

[bradhilton]

Production Qwen tool-call rollouts can return parsed function.arguments as a mapping. The append-only observer accepted that representation, but failed while rebuilding the strict synthetic chat request used to render the completed turn, so the rollout exception was surfaced later as a missing policy history.

Normalize mapping arguments to JSON strings only in observer-owned copies before request validation. This covers both generated tool calls and historical messages that vLLM has shallow-copied and mutated while rendering. The legacy function_call shape follows the same strict request schema. Existing argument strings remain byte-for-byte unchanged, and the original request and generated message remain untouched.

Validation:

• uv run pytest tests/unit/test_append_only.py -q (19 passed)
• uv run ruff check src/art_inference/append_only.py tests/unit/test_append_only.py
• uv run ruff format --check src/art_inference/append_only.py tests/unit/test_append_only.py
• uv run ty check src/art_inference/append_only.py tests/unit/test_append_only.py