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;