feat: make enable_thinking configurable per provider in ai_config.yaml

[ellipse2v]

enable_thinking was hardcoded to False in litellm_client.py (PR #33). This moves it to a per-provider config field so Anthropic extended thinking can be enabled when a sufficient
max_tokens budget is available.

threat_analysis/ai_engine/providers/litellm_client.py

• generate_content(): replace hardcoded False with bool(self.provider_config.get("enable_thinking", False)) — reads from ai_config.yaml, defaults to False
• check_connection(): remove enable_thinking=False — the health ping uses max_tokens=1 and never benefits from reasoning mode

config/ai_config.yaml

• Add enable_thinking as a commented-out field under the mistral provider block, with a note that it should only be set to true alongside a large max_tokens budget

tests/test_litellm_provider.py

• test_enable_thinking_defaults_to_false: asserts acompletion receives enable_thinking=False when the field is absent from provider config
• test_enable_thinking_true_from_provider_config: asserts enable_thinking=True is forwarded to acompletion when explicitly set in provider config
• test_check_connection_does_not_pass_enable_thinking: asserts enable_thinking is absent from the health check call regardless of provider config