feat: add Agents API, Conversations API, and multimodal support (v1.1.0)

Agents API (beta): create, get, update, delete, list agents with tools,
handoffs, completion args, and guardrails support.

Conversations API (beta): create, append, history, messages, restart,
delete, list conversations. Supports agent-backed and model-only
conversations with function calling and handoff execution modes.

Multimodal: ChatMessageContent enum (Text/Parts) with ContentPart
variants for text and image_url. Backwards-compatible constructors.
new_user_message_with_images() for mixed content messages.

Chat: reasoning field on ChatResponseChoice for Magistral models.
HTTP: PATCH methods for agent updates.

81 tests (30 live API integration + 35 serde unit + 16 existing).

tests/v1_constants_test.rs

@@ -32,6 +32,6 @@ fn test_model_constants() {
         expect!(response.object).to_be("chat.completion".to_string());
         expect!(response.choices.len()).to_be(1);
         expect!(response.choices[0].index).to_be(0);
-        expect!(response.choices[0].message.content.len()).to_be_greater_than(0);
+        expect!(response.choices[0].message.content.text().len()).to_be_greater_than(0);
     }
 }