fix(cdp): prune valid_context_ids on executionContextsCleared (#413)

Runtime.executionContextsCleared on navigation told clients every prior
context was gone, but valid_context_ids was insert-only, so a
Runtime.callFunctionOn targeting a pre-navigation context id still ran
instead of being rejected with Chrome's 'Cannot find context with
specified id'. On a long-lived CDP connection the set also grew by one
id per navigation and was never pruned.

Clear valid_context_ids when executionContextsCleared is emitted, then
re-register the default world (id 2) and the isolated worlds as their
executionContextCreated events are emitted. Closes #407.

crates/obscura-cdp/tests/execution_context_pruned_on_navigation.rs

@@ -0,0 +1,69 @@
+// Regression for issue #407: `CdpContext.valid_context_ids` was insert-only,
+// so `Runtime.executionContextsCleared` on navigation never pruned the old
+// context ids. A `Runtime.callFunctionOn` targeting a pre-navigation context
+// still ran (Chrome rejects it with "Cannot find context with specified id"),
+// and the set grew by one id per navigation on a long-lived connection.
+
+use obscura_cdp::dispatch::CdpContext;
+use obscura_cdp::domains::page::emit_navigation_events;
+use obscura_browser::lifecycle::WaitUntil;
+
+fn navigate(ctx: &mut CdpContext, page_id: &str) {
+    emit_navigation_events(
+        ctx,
+        &Some("session-1".to_string()),
+        "frame-1",
+        "loader-1",
+        "http://127.0.0.1/page",
+        page_id,
+        &[],
+        WaitUntil::Load,
+        true,
+    );
+}
+
+#[test]
+fn navigation_prunes_stale_execution_context_ids() {
+    let mut ctx = CdpContext::new();
+    // Seed a stale id that no navigation re-creates (simulates a context from
+    // a prior page state, or simply the pre-navigation main id 1).
+    ctx.valid_context_ids.insert(999);
+
+    navigate(&mut ctx, "page-1");
+
+    // The stale id is gone after executionContextsCleared; the default world
+    // (id 2) and the first isolated world (id 100, counter starts at 100) remain.
+    assert!(
+        !ctx.valid_context_ids.contains(&999),
+        "stale execution context id must be pruned on navigation"
+    );
+    assert!(ctx.valid_context_ids.contains(&2), "default world id 2 must be re-registered");
+    assert!(
+        ctx.valid_context_ids.contains(&100),
+        "isolated world id 100 must be registered: {:?}",
+        ctx.valid_context_ids
+    );
+
+    navigate(&mut ctx, "page-1");
+
+    // Second navigation: the first nav's isolated id (100) is now stale, and a
+    // fresh id (101) takes its place. The set must not grow across navigations.
+    assert!(
+        !ctx.valid_context_ids.contains(&100),
+        "previous navigation's isolated context id must be pruned"
+    );
+    assert!(ctx.valid_context_ids.contains(&101), "fresh isolated id 101 must be registered");
+    assert!(ctx.valid_context_ids.contains(&2), "default world id 2 must survive navigation");
+
+    // Unbounded-growth check: two navigations leave exactly the default world
+    // plus the current isolated world(s), not an accumulating union.
+    let count_after_two_navs = ctx.valid_context_ids.len();
+    navigate(&mut ctx, "page-1");
+    navigate(&mut ctx, "page-1");
+    assert_eq!(
+        ctx.valid_context_ids.len(),
+        count_after_two_navs,
+        "valid_context_ids must not grow across navigations (was {count_after_two_navs}, now {})",
+        ctx.valid_context_ids.len()
+    );
+}