fix!: fail expect.poll when function didn't resolve in time

[hi-ogawa]

Description

• Closes expect.poll(.., { timeout }) doesn't fail early on given timeout #9844
• Closes fix: force poll run to stop at timeout #9848 (supersedes)

This PR makes expect.poll respect timeout properly. If the poll callback or async assertion does not resolve in time, the assertion now fails instead of waiting indefinitely or accepting a late success. Domain snapshot polling already used this stricter model, so this ports the same approach back into generic expect.poll.

This also updates expect.element, which relied on the old timeout behavior. Instead of switching between query and element with magic flag _isLastPollAttempt, it now resolves locators through locator.findElement. toMatchScreenshot still needs special handling but slightly simplified by moving the escape hatch explicitly on expect.poll side.

This is a breaking change because the previous behavior was forgiving: slow callbacks, slow async matchers, and last-attempt-only successes could pass after the configured timeout. Those cases now fail.

TODO

• docs

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[Copilot]

Pull request overview

This PR tightens expect.poll timeout behavior so pending poll callbacks and async assertions fail when the configured timeout expires, and it adapts browser-facing expect.element to that stricter model. In the Vitest codebase, this touches core polling, browser locator assertions, snapshot polling, tests, and user docs.

Changes:

• Reworks expect.poll to race callback/assertion execution against a timeout and expose an AbortSignal to poll callbacks.
• Updates browser expect.element to resolve locators via findElement() instead of the old last-attempt flag flow.
• Adds regression tests and documentation for the stricter timeout/abort behavior.

Reviewed changes

Copilot reviewed 11 out of 12 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
test/unit/test/expect-poll.test.ts	Replaces old last-attempt flag tests with timeout and abort-signal coverage for unresolved callbacks/assertions.
test/snapshots/test/domain-poll.test.ts	Adds snapshot-poll coverage for abort signaling on timeout.
test/browser/test/findElement.test.ts	Adds a browser regression test asserting expect.element now surfaces strict locator behavior.
test/browser/test/expect-element.test.ts	Removes the old test that depended on the previous query-vs-element last-attempt behavior.
packages/vitest/src/types/global.ts	Updates the public expect.poll callback type to receive { signal }.
packages/vitest/src/integrations/chai/poll.ts	Implements the new timeout race logic and timeout-specific poll errors.
packages/snapshot/src/client.ts	Threads abort signaling into domain snapshot polling timeouts.
packages/browser/src/client/tester/expect-element.ts	Switches expect.element locator resolution to findElement() with a shared deadline.
packages/browser/matchers.d.ts	Refreshes expect.element type/docs text to match the new behavior.
package.json	Changes the root test script target from test-core to test-unit.
docs/api/expect.md	Documents whole-operation timeout semantics and the new AbortSignal callback argument.
docs/api/browser/assertions.md	Updates the expect.element docs to describe locator resolution plus poll timeout behavior.