feat: SSEResponse class for streaming Server-Side Events

[michalsn]

Description
This PR introduces SSEResponse, a new response class for streaming Server-Sent Events over HTTP. The class provides event(), comment(), and retry() methods for sending SSE fields to the client. Array data passed to event() is automatically JSON-encoded. The response also handles output buffering, session closing, and appropriate header management.

SSEResponse implements the NonBufferedResponseInterface, which is shared with DownloadResponse. Guard checks in CodeIgniter, PageCache, and the Toolbar that previously referenced DownloadResponse directly now rely on this interface instead. This ensures that both download and SSE responses are correctly excluded from page caching, toolbar injection, and body rendering.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide