fix(cdp): stub setDownloadBehavior, detachFromTarget, activateTarget

These Chrome CDP methods logged "Unknown method" warnings. obscura runs
one page per session and does not download to disk, so there is nothing
to do, but clients that call them should get a success ack instead of a
warning. Added as no-op stubs.

Refs #340.

crates/obscura-cdp/src/dispatch.rs

@@ -220,9 +220,11 @@ fn is_v8_free_method(method: &str) -> bool {
         | "Target.setAutoAttach"
         | "Target.getBrowserContexts" | "Target.createBrowserContext"
         | "Target.disposeBrowserContext" | "Target.getTargetInfo"
+        | "Target.detachFromTarget" | "Target.activateTarget"
         | "Browser.getVersion" | "Browser.close" | "Browser.getWindowForTarget"
         | "Browser.setDownloadBehavior" | "Browser.getWindowBounds" | "Browser.setWindowBounds"
         | "Page.enable" | "Page.disable" | "Page.getFrameTree"
+        | "Page.setDownloadBehavior"
         | "Page.setLifecycleEventsEnabled"
         | "Page.addScriptToEvaluateOnNewDocument" | "Page.removeScriptToEvaluateOnNewDocument"
         | "Page.setInterceptFileChooserDialog" | "Page.getNavigationHistory"