Add a missing tests check and update `CONTRIBUTING.md`

[TahaTesser]

Fixes Prepare repository for tests requirement

Description

• Add a missing tests check will. comment on the PR if the tests are missing
• Added melos command to run tests on the root and samples
• Updated CONTRIBUTING.md for writing tests

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process.

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [x] My PR includes tests for all changed/updated/fixed behaviors.
• [x] All existing and new tests are passing.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] The analyzer (melos run analyze) does not report any problems on my PR.
• [x] The package compiles with the minimum Flutter version stated in the pubspec.yaml

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• [ ] Yes, this is a breaking change.
• [x] No, this is not a breaking change.

[TahaTesser]

"Check for Tests in PR / check-for-tests (pull_request) Failing after 8s" is failing due to permissions on a forked repo PR. See https://github.com/actions/github-script/issues/42#issuecomment-630274929

It would work once the PR is merged. It works fine on my personal repository as seen here