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-cdp/src/dispatch.rs

@@ -39,10 +39,19 @@ impl CdpContext {
     }
 
     pub fn new_with_options(proxy: Option<String>, stealth: bool) -> Self {
-        let default_context = Arc::new(BrowserContext::with_options(
+        Self::new_with_full_options(proxy, stealth, None)
+    }
+
+    pub fn new_with_full_options(
+        proxy: Option<String>,
+        stealth: bool,
+        user_agent: Option<String>,
+    ) -> Self {
+        let default_context = Arc::new(BrowserContext::with_full_options(
             "default".to_string(),
             proxy,
             stealth,
+            user_agent,
         ));
         CdpContext {
             pages: Vec::new(),