feat: add fields endpoint with get, batch-get, and list methods

Implement Field model supporting base fields and polymorphic variants
(list, formula, reference fields via optional properties). Add get_field,
batch_get_fields, and list_fields endpoint methods with tests covering
basic fields, list-type fields, batch retrieval, pagination, and
not-found scenarios.

src/endpoints/fields.rs

@@ -0,0 +1,37 @@
+use reqwest::Method;
+
+use crate::client::OnspringClient;
+use crate::error::Result;
+use crate::models::{CollectionResponse, Field, PagedResponse, PagingRequest};
+
+impl OnspringClient {
+    /// Gets a field by its identifier.
+    pub async fn get_field(&self, field_id: i32) -> Result<Field> {
+        let path = format!("/Fields/id/{}", field_id);
+        self.request(Method::GET, &path, &[], Option::<&()>::None)
+            .await
+    }
+
+    /// Gets up to 100 fields by their identifiers.
+    pub async fn batch_get_fields(&self, ids: &[i32]) -> Result<CollectionResponse<Field>> {
+        self.request(Method::POST, "/Fields/batch-get", &[], Some(&ids))
+            .await
+    }
+
+    /// Gets a paginated list of fields for a given application.
+    pub async fn list_fields(
+        &self,
+        app_id: i32,
+        paging: Option<PagingRequest>,
+    ) -> Result<PagedResponse<Field>> {
+        let path = format!("/Fields/appId/{}", app_id);
+        let mut query = Vec::new();
+        if let Some(p) = paging {
+            query.push(("PageNumber", p.page_number.to_string()));
+            query.push(("PageSize", p.page_size.to_string()));
+        }
+        let query_refs: Vec<(&str, String)> = query.iter().map(|(k, v)| (*k, v.clone())).collect();
+        self.request(Method::GET, &path, &query_refs, Option::<&()>::None)
+            .await
+    }
+}

src/endpoints/mod.rs

@@ -1,2 +1,3 @@
 mod apps;
+mod fields;
 mod ping;

src/models/field.rs

@@ -0,0 +1,35 @@
+use serde::Deserialize;
+use uuid::Uuid;
+
+use super::enums::{FormulaOutputType, Multiplicity};
+
+/// Represents a field in an Onspring application.
+#[derive(Debug, Clone, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct Field {
+    pub id: i32,
+    pub app_id: i32,
+    pub name: Option<String>,
+    #[serde(rename = "type")]
+    pub field_type: Option<String>,
+    pub status: Option<String>,
+    pub is_required: bool,
+    pub is_unique: bool,
+    pub multiplicity: Option<Multiplicity>,
+    pub list_id: Option<i32>,
+    pub values: Option<Vec<ListFieldValue>>,
+    #[serde(rename = "outputType")]
+    pub output_type: Option<FormulaOutputType>,
+    pub referenced_app_id: Option<i32>,
+}
+
+/// Represents a value in a list field.
+#[derive(Debug, Clone, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct ListFieldValue {
+    pub id: Uuid,
+    pub name: String,
+    pub sort_order: i32,
+    pub numeric_value: Option<f64>,
+    pub color: Option<String>,
+}

src/models/mod.rs

@@ -1,7 +1,9 @@
 mod app;
 mod enums;
+mod field;
 mod paging;
 
 pub use app::*;
 pub use enums::*;
+pub use field::*;
 pub use paging::*;