fix(reporter): include failure screenshot attachments in JUnit report

[mixelburg]

Fixes #9904

When browser mode is configured with screenshotFailures: true, screenshots are captured on test failures and stored as internal:failureScreenshot artifacts on the task. However, the JUnit reporter was not referencing these screenshots in its XML output, making it impossible for CI systems to correlate failures with their screenshots.

This PR emits a <system-out> element containing [[ATTACHMENT|/path/to/screenshot.png]] for each failure screenshot artifact on a failed test case. The [[ATTACHMENT|...]] convention is widely supported by CI systems (GitLab, Jenkins, GitHub Actions test report parsers, etc.).

Changes

• packages/vitest/src/node/reporters/junit.ts — after emitting <failure> elements, iterate over task.artifacts for internal:failureScreenshot artifacts and emit their path via <system-out>[[ATTACHMENT|...]]</system-out>
• test/cli/test/reporters/reporters.test.ts — add unit test verifying the [[ATTACHMENT|...]] is emitted in the JUnit output when a failed test has a failure screenshot artifact

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[hi-ogawa]

This shouldn't be internal:failureScreenshot specific, but achieved uniformly through annotation API level https://vitest.dev/guide/test-annotations.html#test-annotations