feat: streaming conversation responses via Mistral API

- Bump mistralai-client to 1.2.0 (conversation streaming support)
- session.rs: add create_or_append_conversation_streaming() — calls
  Mistral with stream:true, emits TextDelta messages to gRPC client
  as SSE chunks arrive, accumulates into ConversationResponse for
  orchestrator tool loop. Handles corrupted conversation recovery
- service.rs: session_chat_via_orchestrator uses streaming variant
- Integration tests: streaming create + append against real Mistral
  API, SSE parsing, accumulate text + function calls

Cargo.toml

@@ -13,7 +13,7 @@ name = "sol"
 path = "src/main.rs"
 
 [dependencies]
-mistralai-client = { version = "1.1.0", registry = "sunbeam" }
+mistralai-client = { version = "1.2.0", registry = "sunbeam" }
 matrix-sdk = { version = "0.9", features = ["e2e-encryption", "sqlite"] }
 opensearch = "2"
 tokio = { version = "1", features = ["full"] }