refactor(llm): Introduce typed EventPart

[JeanMertz]

Replace ConversationEvent inside Event::Part with a purpose-built EventPart enum (Message, Reasoning, Structured, ToolCall(ToolCallPart)), implementing RFD 012. This decouples the streaming transport layer from the persistence layer.

Previously, every streaming chunk allocated a full ConversationEvent with a timestamp and metadata map, even though most of the type went unused during streaming. The EventBuilder also had to silently ignore five EventKind variants (ChatRequest, TurnStart, etc.) that are invalid in a streaming context.

EventBuilder moves from jp_conversation into jp_llm, where it belongs — translating streaming EventParts into persistence ConversationEvents on Flush. Its handle_part signature changes to (usize, EventPart, Map<String, Value>). The persistence layer (ConversationEvent, ConversationStream) is unchanged.

ToolCallRequestAggregator is removed. Providers now emit ToolCallPart::Start { id, name } when a tool call begins, and ToolCallPart::ArgumentChunk(json) for each argument delta. The EventBuilder accumulates these into a complete ToolCallRequest on Flush. AggregationError is removed from the public jp_llm API.

Factory methods on Event (message, reasoning, structured, tool_call_start, tool_call_args) reduce construction boilerplate across all providers and tests. Flush metadata is changed from IndexMap<String, Value> to Map<String, Value> for consistency.

RFD 012 is marked as Implemented.