feat: streaming Conversations API support (v1.2.0)

Add conversation_stream module with full streaming support for the
Mistral Conversations API:

- ConversationEvent enum matching API SSE event types: ResponseStarted,
  MessageOutput (text deltas), FunctionCall, ResponseDone (with usage),
  ResponseError, tool execution, agent handoff events
- parse_sse_line() handles SSE format (skips event: lines, parses
  data: JSON, handles [DONE] and comments)
- accumulate() collects streaming events into a ConversationResponse
- create_conversation_stream_async() and
  append_conversation_stream_async() client methods
- Byte-boundary buffering in sse_to_conversation_events — handles
  JSON split across TCP frames
- Integration tests hit real Mistral API: create stream, append stream,
  stream/non-stream output equivalence

Cargo.toml

@@ -2,7 +2,7 @@
 name = "mistralai-client"
 description = "Mistral AI API client library for Rust (unofficial)."
 license = "Apache-2.0"
-version = "1.1.0"
+version = "1.2.0"
 
 edition = "2021"
 rust-version = "1.76.0"