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/lib.rs

@@ -3,4 +3,4 @@ pub mod dispatch;
 pub mod types;
 pub mod domains;
 
-pub use server::{start, start_with_options};
+pub use server::{start, start_with_full_options, start_with_options};