Update to latest Mistral AI API (v1.0.0)

- Replace closed Model enum with flexible string-based Model type
  with constructor methods for all current models (Mistral Large 3,
  Small 4, Magistral, Codestral, Devstral, Pixtral, Voxtral, etc.)
- Add new API endpoints: FIM completions, Files, Fine-tuning, Batch
  jobs, OCR, Audio transcription, Moderations/Classifications, and
  Agent completions (sync + async for all)
- Add new chat fields: frequency_penalty, presence_penalty, stop,
  n, parallel_tool_calls, reasoning_effort, min_tokens, json_schema
  response format
- Add embedding fields: output_dimension, output_dtype
- Tool parameters now accept raw JSON Schema (serde_json::Value)
  instead of limited enum types
- Add tool call IDs and Required tool choice variant
- Add DELETE HTTP method support and multipart file upload
- Bump thiserror to v2, add reqwest multipart feature
- Remove strum dependency (no longer needed)
- Update all tests and examples for new API

src/v1/chat_stream.rs

@@ -1,7 +1,7 @@
 use serde::{Deserialize, Serialize};
 use serde_json::from_str;
 
-use crate::v1::{chat, common, constants, error};
+use crate::v1::{chat, common, constants, error, tool};
 
 // -----------------------------------------------------------------------------
 // Response
@@ -11,12 +11,11 @@ pub struct ChatStreamChunk {
     pub id: String,
     pub object: String,
     /// Unix timestamp (in seconds).
-    pub created: u32,
+    pub created: u64,
     pub model: constants::Model,
     pub choices: Vec<ChatStreamChunkChoice>,
+    #[serde(skip_serializing_if = "Option::is_none")]
     pub usage: Option<common::ResponseUsage>,
-    // TODO Check this prop (seen in API responses but undocumented).
-    // pub logprobs: ???,
 }
 
 #[derive(Clone, Debug, Deserialize, Serialize)]
@@ -24,14 +23,15 @@ pub struct ChatStreamChunkChoice {
     pub index: u32,
     pub delta: ChatStreamChunkChoiceDelta,
     pub finish_reason: Option<String>,
-    // TODO Check this prop (seen in API responses but undocumented).
-    // pub logprobs: ???,
 }
 
 #[derive(Clone, Debug, Deserialize, Serialize)]
 pub struct ChatStreamChunkChoiceDelta {
     pub role: Option<chat::ChatMessageRole>,
-    pub content: String,
+    #[serde(default)]
+    pub content: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub tool_calls: Option<Vec<tool::ToolCall>>,
 }
 
 /// Extracts serialized chunks from a stream message.
@@ -47,7 +47,6 @@ pub fn get_chunk_from_stream_message_line(
         return Ok(Some(vec![]));
     }
 
-    // Attempt to deserialize the JSON string into ChatStreamChunk
     match from_str::<ChatStreamChunk>(chunk_as_json) {
         Ok(chunk) => Ok(Some(vec![chunk])),
         Err(e) => Err(error::ApiError {