refactor!: modernize core types and client for latest Mistral API

BREAKING CHANGE: Model is now a string-based struct with constructor
methods instead of a closed enum. EmbedModel is removed — use
Model::mistral_embed() instead. Tool parameters now accept
serde_json::Value (JSON Schema) instead of limited enum types.

- Replace Model enum with flexible Model(String) supporting all
  current models: Large 3, Small 4, Medium 3.1, Magistral, Codestral,
  Devstral, Pixtral, Voxtral, Ministral, and arbitrary strings
- Remove EmbedModel enum (consolidated into Model)
- Chat: add frequency_penalty, presence_penalty, stop, n, min_tokens,
  parallel_tool_calls, reasoning_effort, json_schema response format
- Embeddings: add output_dimension and output_dtype fields
- Tools: accept raw JSON Schema, add tool call IDs and Required choice
- Stream delta content is now Option<String> for tool call chunks
- Add Length, ModelLength, Error finish reason variants
- DRY HTTP transport with shared response handlers
- Add DELETE method support and model get/delete endpoints
- Make model_list fields more lenient with Option/default for API compat

src/v1/tool.rs

@@ -1,12 +1,16 @@
 use async_trait::async_trait;
 use serde::{Deserialize, Serialize};
-use std::{any::Any, collections::HashMap, fmt::Debug};
+use std::{any::Any, fmt::Debug};
 
 // -----------------------------------------------------------------------------
 // Definitions
 
 #[derive(Clone, Debug, Eq, PartialEq, Deserialize, Serialize)]
 pub struct ToolCall {
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub id: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub r#type: Option<String>,
     pub function: ToolCallFunction,
 }
 
@@ -22,31 +26,12 @@ pub struct Tool {
     pub function: ToolFunction,
 }
 impl Tool {
+    /// Create a tool with a JSON Schema parameters object.
     pub fn new(
         function_name: String,
         function_description: String,
-        function_parameters: Vec<ToolFunctionParameter>,
+        parameters: serde_json::Value,
     ) -> Self {
-        let properties: HashMap<String, ToolFunctionParameterProperty> = function_parameters
-            .into_iter()
-            .map(|param| {
-                (
-                    param.name,
-                    ToolFunctionParameterProperty {
-                        r#type: param.r#type,
-                        description: param.description,
-                    },
-                )
-            })
-            .collect();
-        let property_names = properties.keys().cloned().collect();
-
-        let parameters = ToolFunctionParameters {
-            r#type: ToolFunctionParametersType::Object,
-            properties,
-            required: property_names,
-        };
-
         Self {
             r#type: ToolType::Function,
             function: ToolFunction {
@@ -63,50 +48,9 @@ impl Tool {
 
 #[derive(Clone, Debug, Deserialize, Serialize)]
 pub struct ToolFunction {
-    name: String,
-    description: String,
-    parameters: ToolFunctionParameters,
-}
-
-#[derive(Clone, Debug, Deserialize, Serialize)]
-pub struct ToolFunctionParameter {
-    name: String,
-    description: String,
-    r#type: ToolFunctionParameterType,
-}
-impl ToolFunctionParameter {
-    pub fn new(name: String, description: String, r#type: ToolFunctionParameterType) -> Self {
-        Self {
-            name,
-            r#type,
-            description,
-        }
-    }
-}
-
-#[derive(Clone, Debug, Deserialize, Serialize)]
-pub struct ToolFunctionParameters {
-    r#type: ToolFunctionParametersType,
-    properties: HashMap<String, ToolFunctionParameterProperty>,
-    required: Vec<String>,
-}
-
-#[derive(Clone, Debug, Deserialize, Serialize)]
-pub struct ToolFunctionParameterProperty {
-    r#type: ToolFunctionParameterType,
-    description: String,
-}
-
-#[derive(Clone, Debug, Eq, PartialEq, Deserialize, Serialize)]
-pub enum ToolFunctionParametersType {
-    #[serde(rename = "object")]
-    Object,
-}
-
-#[derive(Clone, Debug, Eq, PartialEq, Deserialize, Serialize)]
-pub enum ToolFunctionParameterType {
-    #[serde(rename = "string")]
-    String,
+    pub name: String,
+    pub description: String,
+    pub parameters: serde_json::Value,
 }
 
 #[derive(Clone, Debug, Eq, PartialEq, Deserialize, Serialize)]
@@ -127,6 +71,9 @@ pub enum ToolChoice {
     /// The model won't call a function and will generate a message instead.
     #[serde(rename = "none")]
     None,
+    /// The model must call at least one tool.
+    #[serde(rename = "required")]
+    Required,
 }
 
 // -----------------------------------------------------------------------------