fix: Stop dispatching messages after close

[brennanbutler01]

Problem

When several events arrive in one response chunk, calling close() from the first listener still delivers the remaining events with readyState === CLOSED. Reproduced on 4.1.0 (the 4.x release used by the downstream application), published 5.1.1, and current main.

The event dispatch steps require dispatch to stop when the connection is closed. Check the state at the parser's event callback so a listener can close the connection before another buffered event is delivered. No public API changes.

Verification

• Two regressions failed before the fix: ordinary and named events each delivered three messages instead of one. Both now pass, with two controls that receive all three messages while open.
• Tests use a real local HTTP server. The closing cases buffer its response into one chunk so transport fragmentation cannot hide the bug.
• Full Node suite: 48 passed, one existing browser-only skip.
• Full Chromium and desktop WebKit suites: 49 passed each.
• Full Bun 1.3.10 suite: 48 passed, one existing browser-only skip.
• Full workerd suite: 41 passed, seven existing expected failures, and one existing skip. Its console warnings and network-error log also reproduce on untouched main.
• TypeScript 7.0.2 source build, full test/configuration typecheck, and browser type-compatibility check pass. Oxlint, changed-file formatting, and git diff --check pass.
• The built JavaScript export also passes the standalone local-server reproduction.

Verified on macOS with Node 24.13.0, Vitest 4.1.10, and Playwright 1.61.1. Firefox, iOS Safari, Deno, and happy-dom were not run. Local verification reused an isolated toolchain; dependency manifests and lockfiles are unchanged. Includes a patch changeset.

Prepared with OpenAI Codex assistance, including the reproduction, implementation, tests, and local review.

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