Add validation for ref variable in install-wp-tests.sh

[Copilot]

The ref variable is set from WP_TESTS_TAG via parameter expansion but lacks validation before use in download URLs. If WP_TESTS_TAG is malformed (e.g., "tags/", "branches/", or empty), the parameter expansion produces an empty ref, causing download failures with unclear errors.

Changes

• Added empty-string validation for ref after parameter expansion in install_test_suite()
• Added empty-string validation for ref before downloading wp-tests-config-sample.php
• Both validations exit with descriptive error messages including the problematic WP_TESTS_TAG value

Example

# Before: unclear curl/wget failure when ref is empty
download https://raw.githubusercontent.com/.../wordpress-develop/${ref}/wp-tests-config-sample.php

# After: clear error message
if [ -z "$ref" ]; then
    echo -e "${RED}Error:${RESET} Unable to determine git reference from WP_TESTS_TAG: $WP_TESTS_TAG"
    exit 1
fi

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.