only check new links on pull requests to avoid rate limiting (#4522)

Large files like CHANGELOG.md contain hundreds of links. Checking
all of them on every PR triggers HTTP 502 rate limits. On PRs, now
only URLs from added lines in the diff are checked. Pushes to main
still check all links in changed files for full validation.

Mirrors open-telemetry/opentelemetry-python#5162.

Assisted-by: Claude Opus 4.6

Co-authored-by: Lukas Hering <40302054+herin049@users.noreply.github.com>