fix: expand merge commit SHA regex and add SHA-256 test cases

[yaananth]

Fixes https://github.com/github/actions-runtime/issues/5476
Fixes https://github.com/github/actions-runtime/issues/5481
Fixes https://github.com/github/actions-runtime/issues/5482

Summary

• expand merge commit SHA parsing in ref-helper to accept 40- and 64-character SHAs
• add SHA-256 coverage in input-helper and ref-helper tests
• include the coupled input-helper SHA regex update required for the new test coverage

Automated by project-board-agents-plugin via /project-board-agents:lead
Board: https://github.com/orgs/github/projects/24272/views/1
Items: [P2] Fix merge commit SHA regex + [P2] Add SHA-256 test cases
Run: sha256-p2-checkout-00050

[Copilot]

Pull request overview

Expands SHA parsing to support repositories using SHA-256 (64-hex) commit IDs, and adds test coverage to ensure SHA-256 refs and merge commit messages are handled correctly.

Changes:

• Update input-helper to treat 64-hex refs as explicit SHAs (set commit, clear ref).
• Update ref-helper merge commit message parsing to accept both 40- and 64-hex SHAs.
• Add Jest coverage for SHA-256 inputs and SHA-256 merge commit message parsing (including a negative case).

Show a summary per file

File	Description
src/ref-helper.ts	Expands merge commit message SHA regex to accept 40- or 64-char hex SHAs.
src/input-helper.ts	Expands explicit-SHA detection to accept 40- or 64-char hex SHAs.
dist/index.js	Updates bundled output to reflect the new SHA regex behavior.
test/ref-helper.test.ts	Adds SHA-256 checkout info test and new checkCommitInfo SHA-1/SHA-256/invalid-merge coverage.
test/input-helper.test.ts	Adds SHA-256 explicit SHA input test ensuring ref is cleared and commit is set.

Copilot's findings

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• Files reviewed: 4/5 changed files
• Comments generated: 0