Fix repeat contributor detection

[GodHad]

Description

This updates first-time contributor detection to use GitHub search result counts instead of checking only the first page returned by issues.listForRepo.

The previous implementation listed repository issues/PRs created by the author and then checked whether the filtered first page had exactly one result. For contributors with older prior activity beyond the first page, this could incorrectly treat them as first-time contributors.

Motivation and Context

Fixes #16

This change prevents the welcome action from occasionally commenting on issues or pull requests opened by repeat contributors, veteran contributors, or maintainers.

Then it only posts the welcome comment when total_count === 1.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Enhancement (changes that improvement of current feature or performance)
• Refactoring (changes that neither fixes a bug nor adds a feature)
• Test Case (changes that add missing tests or correct existing tests)
• Code style optimization (changes that do not affect the meaning of the code)
• Docs (changes that only update documentation)
• Chore (changes that don't modify src or test files)

Self Check before Merge

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.