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

examples/fim.rs

@@ -0,0 +1,21 @@
+use mistralai_client::v1::{
+    client::Client,
+    constants::Model,
+    fim::FimParams,
+};
+
+fn main() {
+    // This example suppose you have set the `MISTRAL_API_KEY` environment variable.
+    let client = Client::new(None, None, None, None).unwrap();
+
+    let model = Model::codestral_latest();
+    let prompt = "def fibonacci(n):".to_string();
+    let options = FimParams {
+        suffix: Some("\n    return result".to_string()),
+        temperature: Some(0.0),
+        ..Default::default()
+    };
+
+    let response = client.fim(model, prompt, Some(options)).unwrap();
+    println!("Completion: {}", response.choices[0].message.content);
+}