Avoid validate workflow failures on forked pull requests

[Copilot]

The validate job was failing after successful SHA checks because the summary step always attempted to post a PR comment. On forked pull_request runs, that write uses a read-only token and fails with Resource not accessible by integration.

• Summary publishing

  • Move the SHA validation report to the job summary unconditionally via core.summary.
  • Keep the existing summary content and per-stack status output intact.

• Fork-safe PR commenting

  • Detect whether the PR head repo differs from the base repo.
  • Only post an issue comment for same-repo PRs.
  • Skip comment creation for forked PRs instead of failing the job.

• Failure semantics

  • Preserve the existing behavior where the job still fails if PHP or .NET SHA validation fails.
  • Limit the change to the reporting path; validation logic itself is unchanged.

const pullRequest = context.payload.pull_request;
const isFork = pullRequest?.head?.repo?.full_name !== pullRequest?.base?.repo?.full_name;

await core.summary.addRaw(body, true).write();

if (!isFork && context.issue.number) {
  await github.rest.issues.createComment({ ... });
}