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.lock

@@ -828,6 +828,15 @@ dependencies = [
  "cfg-if",
 ]
 
+[[package]]
+name = "crossbeam-channel"
+version = "0.5.15"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "82b8f8f868b36967f9606790d1903570de9ceaf870a7bf9fbbd3016d636a2cb2"
+dependencies = [
+ "crossbeam-utils",
+]
+
 [[package]]
 name = "crossbeam-deque"
 version = "0.8.6"
@@ -1357,7 +1366,7 @@ dependencies = [
  "libc",
  "option-ext",
  "redox_users",
- "windows-sys 0.61.2",
+ "windows-sys 0.59.0",
 ]
 
 [[package]]
@@ -1482,7 +1491,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "39cab71617ae0d63f51a36d69f866391735b51691dbda63cf6f96d042b63efeb"
 dependencies = [
  "libc",
- "windows-sys 0.61.2",
+ "windows-sys 0.59.0",
 ]
 
 [[package]]
@@ -3070,9 +3079,9 @@ dependencies = [
 
 [[package]]
 name = "mistralai-client"
-version = "1.1.0"
+version = "1.2.0"
 source = "sparse+https://src.sunbeam.pt/api/packages/studio/cargo/"
-checksum = "6a17c5600508f30965a8e6ab78e947a0db7017edef8ffb022aa9ac71f50ebaff"
+checksum = "24258b2ba72432f9c147a5a80e6d862c7b3c43db859135c619c948cf6c9e4fdf"
 dependencies = [
  "async-stream",
  "async-trait",
@@ -3154,7 +3163,7 @@ version = "0.50.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7957b9740744892f114936ab4a57b3f487491bbeafaf8083688b16841a4240e5"
 dependencies = [
- "windows-sys 0.61.2",
+ "windows-sys 0.59.0",
 ]
 
 [[package]]
@@ -3777,7 +3786,7 @@ dependencies = [
  "once_cell",
  "socket2",
  "tracing",
- "windows-sys 0.60.2",
+ "windows-sys 0.59.0",
 ]
 
 [[package]]
@@ -4290,7 +4299,7 @@ dependencies = [
  "errno",
  "libc",
  "linux-raw-sys 0.12.1",
- "windows-sys 0.61.2",
+ "windows-sys 0.59.0",
 ]
 
 [[package]]
@@ -4736,6 +4745,7 @@ dependencies = [
  "tonic-prost",
  "tonic-prost-build",
  "tracing",
+ "tracing-appender",
  "tracing-subscriber",
  "tree-sitter",
  "tree-sitter-python",
@@ -5355,7 +5365,7 @@ dependencies = [
  "getrandom 0.4.2",
  "once_cell",
  "rustix 1.1.4",
- "windows-sys 0.61.2",
+ "windows-sys 0.59.0",
 ]
 
 [[package]]
@@ -5838,6 +5848,18 @@ dependencies = [
  "tracing-core",
 ]
 
+[[package]]
+name = "tracing-appender"
+version = "0.2.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "786d480bce6247ab75f005b14ae1624ad978d3029d9113f0a22fa1ac773faeaf"
+dependencies = [
+ "crossbeam-channel",
+ "thiserror 2.0.18",
+ "time",
+ "tracing-subscriber",
+]
+
 [[package]]
 name = "tracing-attributes"
 version = "0.1.31"
@@ -6425,7 +6447,7 @@ version = "0.1.11"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "c2a7b1c03c876122aa43f3020e6c3c3ee5c05081c9a00739faf7503aeba10d22"
 dependencies = [
- "windows-sys 0.61.2",
+ "windows-sys 0.59.0",
 ]
 
 [[package]]