fix: stop dispatching SSE events after close

[zubairchowdhury888-art]

Summary

When multiple SSE events arrive in a single buffered chunk, calling
EventSource.close() from the first event listener previously allowed
subsequent buffered events to continue dispatching.

This change adds a closed-state guard to stop event dispatch once the
EventSource has reached CLOSED.

Fixes #362

Regression test

Added a regression test where:

• first, second, and third events arrive in a single chunk
• the EventSource is closed from the first event listener
• only the first event is expected to be dispatched

The unmodified upstream implementation reproduced the issue:

messages dispatched: expected 3 to be 1

The same test passes after the fix.

Verification

• Node: 45 passed, 1 skipped
• Targeted regression: 1 passed, 45 skipped
• Lint/typecheck: 0 warnings, 0 errors
• happy-dom: 42 passed, 4 expected-fail
• workerd: 1 failure, 37 passed, 7 expected-fail; the failure was verified as pre-existing against upstream
• Browser suite could not run because the Playwright/Vitest browser configuration crashes during CLI startup in this environment
• Bun/Deno/iOS were not run because those runtimes are not installed

Changes

• src/EventSource.ts — closed-state guard
• test/client.test.ts — regression test
• .changeset/stop-dispatch-after-close.md — patch changeset

No public API change and no unrelated refactoring.

The implementation is intentionally minimal: it prevents further event dispatch when the EventSource is already CLOSED.