Update test suite to ensure 100% code coverage

[clue]

This changeset updates the CI setup to ensure 100% code coverage.

Builds on top of #67 and clue/reactphp-eventsource#35

[Copilot]

Pull request overview

Updates the CI pipeline and documentation to enforce and advertise 100% statement coverage for this library across the PHP test matrix.

Changes:

• Generates Clover coverage output in CI and adds a step to fail the job unless coverage is 100%.
• Adds @codeCoverageIgnore annotations to exclude a couple of hard-to-cover statements from coverage metrics.
• Updates README with a 100% coverage badge and local coverage-report instructions.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/ProxyConnector.php	Adds @codeCoverageIgnore markers to exclude specific statements from coverage.
README.md	Documents 100% coverage goal and adds a static 100% coverage badge plus local coverage command.
.github/workflows/ci.yml	Produces clover.xml and enforces 100% statement coverage via a PHP check step.

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