Improve error reporting for invalid responses by using ResponseException

[clue]

This changeset improves error reporting when the EventSource connection receives an invalid response (non-200 status code or invalid Content-Type header). The first commit improves error messages to be more descriptive, the second commit switches to ResponseException which provides access to the response object.

Builds on top of #14, #2, #32 and others, follows similar approach as clue/framework-x#284

[Copilot]

Pull request overview

This PR improves permanent error reporting for EventSource when the initial HTTP response is invalid (non-200 status or invalid/missing Content-Type), switching emitted errors to React\Http\Message\ResponseException so consumers can access the underlying response.

Changes:

• Emit ResponseException with descriptive, safely-quoted error messages for invalid status codes and invalid Content-Type.
• Add a small internal quote() helper to truncate/escape response details in error messages.
• Update tests to cover multiple invalid status/content-type cases via data providers and assert ResponseException behavior; update README accordingly.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/EventSource.php	Emits ResponseException for permanent failures and adds quoting helper for safe, bounded error details.
tests/EventSourceTest.php	Expands assertions and cases (status + content-type) to validate new exception type/message/response access.
README.md	Fixes example variable name and documents ResponseException behavior for permanent failures.

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