feat(client): add RFC 2387 multipart/related support

Add runtime support for multipart/related requests following RFC 2387.
Introduces MultipartPart struct, MultipartRelatedBody trait, and
RequestBuilderExt::multipart_related() method for constructing
properly formatted multipart/related request bodies.

The implementation includes unique boundary generation using timestamp
and atomic counter, proper Content-Type and Content-ID headers for each
part, and correct CRLF formatting per RFC 2046/2387.

This provides the runtime foundation for multipart file uploads with
metadata in a single request body.

Refs: #1240

Author: lanej

progenitor-client/src/progenitor_client.rs

@@ -526,9 +526,22 @@ pub fn encode_path(pc: &str) -> String {
     percent_encoding::utf8_percent_encode(pc, PATH_SET).to_string()
 }
 
+#[doc(hidden)]
+pub struct MultipartPart<'a> {
+    pub content_type: &'static str,
+    pub content_id: &'a str,
+    pub bytes: Vec<u8>,
+}
+
+#[doc(hidden)]
+pub trait MultipartRelatedBody {
+    fn as_multipart_parts(&self) -> Vec<MultipartPart<'_>>;
+}
+
 #[doc(hidden)]
 pub trait RequestBuilderExt<E> {
     fn form_urlencoded<T: Serialize + ?Sized>(self, body: &T) -> Result<RequestBuilder, Error<E>>;
+    fn multipart_related<T: MultipartRelatedBody + ?Sized>(self, body: &T) -> Result<RequestBuilder, Error<E>>;
 }
 
 impl<E> RequestBuilderExt<E> for RequestBuilder {
@@ -543,6 +556,62 @@ impl<E> RequestBuilderExt<E> for RequestBuilder {
                     .map_err(|_| Error::InvalidRequest("failed to serialize body".to_string()))?,
             ))
     }
+
+    fn multipart_related<T: MultipartRelatedBody + ?Sized>(self, body: &T) -> Result<Self, Error<E>> {
+        // Generate a unique boundary
+        let boundary = generate_multipart_boundary();
+
+        // Get the parts from the body
+        let parts = body.as_multipart_parts();
+
+        let mut body_bytes = Vec::new();
+
+        // Write each part
+        for part in parts {
+            // Write boundary
+            body_bytes.extend_from_slice(format!("--{}\r\n", boundary).as_bytes());
+
+            // Write Content-Type header
+            body_bytes.extend_from_slice(format!("Content-Type: {}\r\n", part.content_type).as_bytes());
+
+            // Write Content-ID header
+            body_bytes.extend_from_slice(format!("Content-ID: <{}>\r\n\r\n", part.content_id).as_bytes());
+
+            // Write content bytes
+            body_bytes.extend_from_slice(&part.bytes);
+
+            // Write CRLF after content
+            body_bytes.extend_from_slice(b"\r\n");
+        }
+
+        // Write closing boundary
+        body_bytes.extend_from_slice(format!("--{}--\r\n", boundary).as_bytes());
+
+        // Set Content-Type header with boundary
+        let content_type = format!("multipart/related; boundary={}; type=application/json", boundary);
+
+        Ok(self
+            .header(
+                reqwest::header::CONTENT_TYPE,
+                reqwest::header::HeaderValue::from_str(&content_type)
+                    .map_err(|e| Error::InvalidRequest(format!("invalid content-type header: {}", e)))?,
+            )
+            .body(body_bytes))
+    }
+}
+
+fn generate_multipart_boundary() -> String {
+    use std::sync::atomic::{AtomicU64, Ordering};
+    static COUNTER: AtomicU64 = AtomicU64::new(0);
+
+    // Generate a unique boundary using timestamp and counter
+    let count = COUNTER.fetch_add(1, Ordering::SeqCst);
+    let timestamp = std::time::SystemTime::now()
+        .duration_since(std::time::UNIX_EPOCH)
+        .map(|d| d.as_millis())
+        .unwrap_or(0);
+
+    format!("progenitor_boundary_{}_{}", timestamp, count)
 }
 
 #[doc(hidden)]