Fix version padding for GitHub tag downloads in install-wp-tests.sh

[Copilot]

The WordPress version-check API returns versions in major.minor format (e.g., 6.9), but GitHub tags require the full semantic version including patch (e.g., 6.9.0). This causes download failures when installing the latest WordPress version.

Changes:

• Added version padding logic to append .0 to major.minor versions before constructing GitHub tag URLs
• Applied only in the latest version path where the version-check API is queried

Example:

# Before: API returns "6.9" → downloads from .../tags/6.9.tar.gz (404)
# After:  API returns "6.9" → appends ".0" → downloads from .../tags/6.9.0.tar.gz (success)

if [[ $LATEST_VERSION =~ ^[0-9]+\.[0-9]+$ ]]; then
    LATEST_VERSION="${LATEST_VERSION}.0"
fi

Addresses feedback from #356.

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