Fix HTTP/2 requests failing with duplicate Content-Length

When Yaak moved compression, redirect, and multipart handling out of
reqwest, it started explicitly adding Content-Length as an HTTP header.
Over HTTP/2, hyper also sets content-length internally via DATA frame
sizing, causing a duplicate that HTTP/2 servers (like Node.js) reject
with PROTOCOL_ERROR.

Instead of setting Content-Length as an explicit header, carry the
content length through SendableBody::Stream and use http_body::Body's
size_hint() to let hyper handle it automatically for both HTTP/1.1
and HTTP/2.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/yaak-http/tests/h2_repro.rs

@@ -0,0 +1,114 @@
+/// Integration test to reproduce the HTTP/2 "HTTP/1 specific headers are forbidden" bug.
+///
+/// Requires a local HTTP/2-only server running on https://localhost:8443
+/// Start one with: node h2server.mjs (see tmp dir for the script)
+///
+/// Run with: cargo test -p yaak-http --test h2_repro -- --nocapture
+use reqwest::redirect;
+use tokio::sync::mpsc;
+use yaak_http::sender::{HttpSender, ReqwestSender};
+use yaak_http::types::SendableHttpRequest;
+use yaak_tls::get_tls_config;
+
+fn build_yaak_client() -> reqwest::Client {
+    let tls_config = get_tls_config(false, true, None).unwrap();
+    reqwest::Client::builder()
+        .redirect(redirect::Policy::none())
+        .no_gzip()
+        .no_brotli()
+        .no_deflate()
+        .referer(false)
+        .tls_info(true)
+        .pool_max_idle_per_host(0)
+        .use_preconfigured_tls(tls_config)
+        .build()
+        .unwrap()
+}
+
+async fn send_and_print(name: &str, request: SendableHttpRequest) {
+    println!("\n=== {} ===", name);
+    let sender = ReqwestSender::with_client(build_yaak_client());
+    let (event_tx, mut event_rx) = mpsc::channel(100);
+
+    let test_name = name.to_string();
+    let handle = tokio::spawn(async move {
+        while let Some(event) = event_rx.recv().await {
+            println!("  [{}] {}", test_name, event);
+        }
+    });
+
+    match sender.send(request, event_tx).await {
+        Ok(response) => {
+            println!("  [{}] OK: HTTP {} {:?}", name, response.status, response.version);
+        }
+        Err(e) => {
+            println!("  [{}] FAILED: {}", name, e);
+        }
+    }
+    let _ = handle.await;
+}
+
+#[tokio::test]
+async fn test_h2_post_with_body() {
+    // Narrow down: POST with body but no extra headers
+    send_and_print("post-body-no-headers", SendableHttpRequest {
+        url: "https://localhost:8443/".to_string(),
+        method: "POST".to_string(),
+        headers: vec![],
+        body: Some(yaak_http::types::SendableBody::Bytes(
+            bytes::Bytes::from(r#"{"hello":"world"}"#),
+        )),
+        options: Default::default(),
+    }).await;
+
+    // POST with body + content-type only
+    send_and_print("post-body-content-type", SendableHttpRequest {
+        url: "https://localhost:8443/".to_string(),
+        method: "POST".to_string(),
+        headers: vec![
+            ("Content-Type".to_string(), "application/json".to_string()),
+        ],
+        body: Some(yaak_http::types::SendableBody::Bytes(
+            bytes::Bytes::from(r#"{"hello":"world"}"#),
+        )),
+        options: Default::default(),
+    }).await;
+
+    // POST with body + content-length only
+    send_and_print("post-body-content-length", SendableHttpRequest {
+        url: "https://localhost:8443/".to_string(),
+        method: "POST".to_string(),
+        headers: vec![
+            ("Content-Length".to_string(), "18".to_string()),
+        ],
+        body: Some(yaak_http::types::SendableBody::Bytes(
+            bytes::Bytes::from(r#"{"hello":"world"}"#),
+        )),
+        options: Default::default(),
+    }).await;
+
+    // POST with body + all typical Yaak headers
+    send_and_print("post-body-all-yaak-headers", SendableHttpRequest {
+        url: "https://localhost:8443/".to_string(),
+        method: "POST".to_string(),
+        headers: vec![
+            ("Content-Type".to_string(), "application/json".to_string()),
+            ("Content-Length".to_string(), "18".to_string()),
+            ("User-Agent".to_string(), "yaak".to_string()),
+            ("Accept".to_string(), "*/*".to_string()),
+        ],
+        body: Some(yaak_http::types::SendableBody::Bytes(
+            bytes::Bytes::from(r#"{"hello":"world"}"#),
+        )),
+        options: Default::default(),
+    }).await;
+
+    // GET with no body (control — should work)
+    send_and_print("get-control", SendableHttpRequest {
+        url: "https://localhost:8443/".to_string(),
+        method: "GET".to_string(),
+        headers: vec![],
+        body: None,
+        options: Default::default(),
+    }).await;
+}