Update integration tests to use extended grep instead of PCRE

[clue]

This changeset updates the integration tests to use the more widely supported extended grep instead of PCRE. Among others, this makes it easier to run these tests across different platforms, including macOS and Alpine Linux.

Builds on top of #256, #3 and others
Refs #209 and others

[Copilot]

Pull Request Overview

This PR replaces PCRE (Perl-Compatible Regular Expressions) with extended grep to improve cross-platform compatibility for integration tests. The change enables tests to run more reliably across different platforms including macOS and Alpine Linux.

• Replaces PCRE flag -P with extended grep flag -E across all test assertions
• Updates regex patterns from [\r\n] to $ for end-of-line matching
• Adds sed command to strip carriage returns from curl output

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