ci: add contributor check to validate-files-and-commits

[Anshumancanrock]

Description

When a PR author isn't listed in .github/contributors.yaml, the nubificus/git-trailers action fails at merge time after the review is already done. This happens in both add-git-trailers.yml and pr-merge.yml.

This PR adds a check-contributor job to validate-files-and-commits.yml that catches missing contributors early during CI.

The job normalizes the PR author's username using the same sed logic as nubificus/git-trailers (strips [bot] suffix, app/ prefix), does a primary lookup by YAML key, and falls back to a name field search for bot accounts. If the author isn't found, it fails with a message showing exactly what entry to add.

Related issues

• Fixes Add a worfklow to check if contributor exists in contributors.yaml #683

How was this tested?

Tested the grep matching logic locally against the current contributors.yaml:

• Normal users (ananos, johnp41) : matched correctly
• Hyphenated names (mgkeka-nbfc) : matched correctly
• Case-sensitive entries (DionisiaK4) : matched correctly
• Bot accounts (dependabot[bot]) : matched via bot fallback
• Non-existent users : correctly rejected
• Partial username matches (anano vs ananos) : correctly rejected
• Empty input : correctly rejected

LLM usage

N/a

Checklist

• I have read the contribution guide.
• The linter passes locally (make lint).
• The e2e tests of at least one tool pass locally (make test_ctr, make test_nerdctl, make test_docker, make test_crictl).
• If LLMs were used: I have read the llm policy.

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[Anshumancanrock]

Hi @cmainas, please take a look at this PR and let me know your thoughts.