fix(serve): plumb --user-agent through CDP server (#71) (#76)

obscura serve --user-agent <ua> only logged the value via tracing::info!
and never passed it to the CDP server, so every navigation kept using
the default Chrome UA string and httpbin.org/user-agent reported the
default rather than the override.

Plumb the option end-to-end:

- BrowserContext::with_full_options(id, proxy, stealth, user_agent)
  initializes both BrowserContext::user_agent and the underlying
  ObscuraHttpClient::user_agent RwLock at construction time so the
  first navigation already sees the override.
- CdpContext::new_with_full_options forwards the UA to the default
  BrowserContext.
- obscura_cdp::start_with_full_options accepts the UA and passes it
  through cdp_processor.
- main.rs Serve arm calls start_with_full_options with the parsed UA.
- run_multi_worker_serve forwards --user-agent <ua> when spawning
  worker subprocesses so multi-worker mode honors the flag too.

Existing two-arg start_with_options and three-arg with_options stay
as thin shims (call the full variant with None) so older callers and
the public crate API remain source-compatible.

Three new unit tests in obscura-browser cover the propagation, the
Chrome default fallback, and the legacy with_options path.

Generated by Ora Studio
Vibe coded by ousamabenyounes

Co-authored-by: Ora Studio <[email protected]>

crates/obscura-browser/src/context.rs

@@ -30,6 +30,15 @@ impl BrowserContext {
     }
 
     pub fn with_options(id: String, proxy_url: Option<String>, stealth: bool) -> Self {
+        Self::with_full_options(id, proxy_url, stealth, None)
+    }
+
+    pub fn with_full_options(
+        id: String,
+        proxy_url: Option<String>,
+        stealth: bool,
+        user_agent: Option<String>,
+    ) -> Self {
         let cookie_jar = Arc::new(CookieJar::new());
         let mut client = ObscuraHttpClient::with_options(
             cookie_jar.clone(),
@@ -38,12 +47,21 @@ impl BrowserContext {
         if stealth {
             client.block_trackers = true;
         }
+        let resolved_ua = user_agent.unwrap_or_else(|| {
+            "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/145.0.0.0 Safari/537.36".to_string()
+        });
+        // Sync the http client's UA at construction so navigation requests pick it
+        // up before any async setup runs. The lock has no other holders here, so
+        // try_write always succeeds; we fall back silently if it ever fails.
+        if let Ok(mut guard) = client.user_agent.try_write() {
+            *guard = resolved_ua.clone();
+        }
         let http_client = Arc::new(client);
         BrowserContext {
             id,
             cookie_jar,
             http_client,
-            user_agent: "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/145.0.0.0 Safari/537.36".to_string(),
+            user_agent: resolved_ua,
             proxy_url,
             robots_cache: Arc::new(RobotsCache::new()),
             obey_robots: false,
@@ -55,3 +73,41 @@ impl BrowserContext {
         Self::with_options(id, proxy_url, false)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[tokio::test(flavor = "current_thread")]
+    async fn with_full_options_propagates_user_agent_to_http_client() {
+        let ctx = BrowserContext::with_full_options(
+            "test".to_string(),
+            None,
+            false,
+            Some("Custom-UA/1.0".to_string()),
+        );
+        assert_eq!(ctx.user_agent, "Custom-UA/1.0");
+        let client_ua = ctx.http_client.user_agent.read().await.clone();
+        assert_eq!(client_ua, "Custom-UA/1.0");
+    }
+
+    #[tokio::test(flavor = "current_thread")]
+    async fn with_full_options_falls_back_to_chrome_default() {
+        let ctx = BrowserContext::with_full_options(
+            "test".to_string(),
+            None,
+            false,
+            None,
+        );
+        assert!(ctx.user_agent.contains("Chrome"));
+        let client_ua = ctx.http_client.user_agent.read().await.clone();
+        assert!(client_ua.contains("Chrome"));
+        assert_eq!(ctx.user_agent, client_ua);
+    }
+
+    #[tokio::test(flavor = "current_thread")]
+    async fn with_options_keeps_default_user_agent() {
+        let ctx = BrowserContext::with_options("test".to_string(), None, false);
+        assert!(ctx.user_agent.contains("Chrome"));
+    }
+}