bun test: do not focus a file on a .only that has no test to run - #43239

Repro. A fuzzer that composes describe and test modifiers found it. There is no GitHub issue.

import { test, describe } from "bun:test";
test("a1 plain top-level test", () => console.log("a1 ran"));
describe.skip("skipped group", () => {
  test.only("a2 only, inside describe.skip", () => console.log("a2 ran"));
});

Before: (skip) skipped group > a2 ..., 0 pass, 1 skip, Ran 1 test, exit 0, and a1 ran never prints. After: a1 ran, (pass) a1 ..., (skip) skipped group > a2 ..., 1 pass, 1 skip, Ran 2 tests.

The symptom exists only where bun does not detect CI. With CI=true, .only throws (.only is disabled in CI environments), before and after this PR.

Regression status, per face (I ran 1.2.21, 1.2.22 and 1.2.23):

• The reported face was never correct. 1.2.21 and 1.2.22 also print 0 pass, 1 skip. The same holds for a describe.only with only skipped tests and for an empty describe.only.
• The two faces inside a describe.only are regressions from the test runner rewrite (Rewrite test/describe, add test.concurrent #22534, first released in 1.2.23). With a nested describe.skip that holds a test.only, or a nested describe.only that holds only skipped tests, 1.2.22 runs the outer plain test. 1.2.23 and the current canary drop it.

What the runners do (I ran each case, Jest 29.7 and Vitest 3.2.7):

Vitest focuses the file in the first case and runs nothing, but it reports the plain test as skipped (1 skipped (2)).

How the walk works: Only::Contains is closed upward, because mark_contains_only always marks up to the root. So the walk from a test stops at the first scope that is not No. If that scope is Yes, its ancestors get marked (an outer describe.only turns into Contains, which is the existing only-inside-only rule). If it is Contains, the ancestors are marked already. The result does not depend on the order of the declarations.

Review follow-ups (second commit):

• -t: ScopeFunctions used to replace a test's own skip or todo mode with FilteredOut before the entry was created, so under -t a skipped test looked able to run and focused the file. It now passes the real callback and filtered_out. append_test decides focus from the mode before the filter and then sets FilteredOut. A test in a skip or todo describe keeps that mode, as before (BaseScope::inherited_mode holds that rule for both places). The output of a -t run over skip, todo and filtered tests is identical to main.
• --todo: a todo test without a body cannot run, so it does not focus the file.
• A describe.only whose callback throws before it declares a test keeps the file focused, as on main. Collection::handle_uncaught_exception calls mark_focus on the failed describe. The test for it fails when that call is removed.

Output change that follows: next to a test.only that runs, a test.only nested in a describe.skip is now dropped like any other test that is not focused. Before, it was listed as (skip). For the same reason bun test --only on the repro file now reports Ran 0 tests.

Not changed:

• The innermost .only wins across describe levels. describe.only { plain, describe.only { test } } runs only the inner test on main and on this PR (the rule that only-inside-only.test.ts pins). Jest 29.7 and bun 1.2.22 run plain too. Whether to restore that is a separate question.
• describe.skip > describe.only > test: Jest runs that test, because an explicit only on the inner block overrides the inherited skip. In bun a child of a skipped scope is always skipped, so the test is reported as skip and the rest of the file runs.
• test.only plus a -t <pattern> that does not match it: the file stays focused and nothing else runs, as in Jest. bun test exits 1 with matched 0 tests when no test in the run matches, so that case is not a silent pass.

Suites run on the debug build: test-only, only-inside-only, bun_test, describe, nested-describes, ci-restrictions, jest-hooks, test-test, concurrent, dots, node-test, test/cli/test/bun-test.test.ts, parallel, pass-with-no-tests, test-filter-lifecycle-snapshot, test/cli/inspect/test-reporter.test.ts, and the regression tests whose fixtures use .only, -t, todo or skip (5961, 14135, 19875, 20092, 08964, 21177, 08768, 25903, 3657, 5228, 20753).

This PR conflicts with #40235 (remove unsafe from the test runner) in bun_test.rs, Collection.rs and ScopeFunctions.rs. mark_focus adds one more parent walk in the style of mark_contains_only. On top of #40235 it becomes a loop over parent.upgrade(). I will rebase whichever lands second.

bun_test.rs sits behind the cfg_jsc! macro in test_runner/mod.rs, so cargo fmt does not reach it. The new code follows the file's existing style.