feat: add files endpoint with info, download, upload, and delete

Implement FileInfo, FileResponse, SaveFileRequest, and
CreatedWithIdResponse models. Add get_file_info, get_file (binary
download), upload_file (multipart), and delete_file methods. Tests
cover metadata retrieval, binary content download with headers, file
upload, deletion, and not-found error with message extraction.

src/models/file.rs

@@ -0,0 +1,45 @@
+use bytes::Bytes;
+use chrono::{DateTime, Utc};
+use serde::Deserialize;
+
+/// Information about a file attachment.
+#[derive(Debug, Clone, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct FileInfo {
+    #[serde(rename = "type")]
+    pub file_type: Option<String>,
+    pub content_type: Option<String>,
+    pub name: Option<String>,
+    pub created_date: Option<DateTime<Utc>>,
+    pub modified_date: Option<DateTime<Utc>>,
+    pub owner: Option<String>,
+    pub notes: Option<String>,
+    pub file_href: Option<String>,
+}
+
+/// The content of a downloaded file.
+#[derive(Debug, Clone)]
+pub struct FileResponse {
+    pub content_type: Option<String>,
+    pub file_name: Option<String>,
+    pub data: Bytes,
+}
+
+/// Request to upload a file.
+#[derive(Debug, Clone)]
+pub struct SaveFileRequest {
+    pub record_id: i32,
+    pub field_id: i32,
+    pub notes: Option<String>,
+    pub modified_date: Option<DateTime<Utc>>,
+    pub file_name: String,
+    pub file_data: Vec<u8>,
+    pub content_type: String,
+}
+
+/// Response from creating a file, containing the new file ID.
+#[derive(Debug, Clone, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct CreatedWithIdResponse {
+    pub id: i32,
+}