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

[datamweb]

First of all, I’m really happy to see CodeIgniter continuing its path towards modernization.

I had a couple of thoughts regarding the example provided in the documentation. While the current example is excellent from a PHP developer’s perspective and clearly demonstrates the API usage, I believe it may be less impactful for developers who are focused on building end‑user products.

Perhaps a more product‑oriented example could better showcase the real value of this class — for example, a small notification system or a real‑time update scenario. This could help readers more intuitively grasp the practical power and use cases enabled by this addition.

[datamweb]

formatMultiline() and write() appear to be natural extension
boundaries (I/O and formatting).
Would making them protected help avoid duplication or framework
forking when custom SSE behavior is needed?

Suggested change

	private function formatMultiline(string $prefix, string $value): string
	protected function formatMultiline(string $prefix, string $value): string

[michalsn]

Formatting methods are dictated by the spec, not by application needs. I don't see a scenario where this would need modification.

[datamweb]

Suggested change

	private function write(string $output): bool
	protected function write(string $output): bool

[michalsn]

Again, I can't imagine a legitimate scenario where this may be handy. If a concrete need comes up later, promoting private to protected is a non-breaking change. Going the other direction isn't.

[michalsn]

@datamweb Thanks for the feedback. I've added a "live notifications" example to better illustrate the idea behind SSE.

[datamweb]

Thank you for addressing this so thoroughly.
Let's see what others say!