Add tests for the new bun ecosystem

[markhallen]

What are you trying to accomplish?

As part of our work to support bun in its own ecosystem we are adding specs to cover the functionality.

Part of #6528

The specs revealed that we should update the regex used in bun/lib/dependabot/bun/file_updater.rb

Added specs for:

• BunPackageManager
• FileFetcher
• FileParser::LockfileParser
• FileParser
• FileUpdater
• Requirement

Ans required fixtures.

Anything you want to highlight for special attention from reviewers?

Check that coverage is adequate.

How will you know you've accomplished your goal?

The regex used by the updater doesn't appear to causes issues as full testing has already taken place and produced the expected PRs. We should pay attention for any adverse reactions though.

Checklist

• I have run the complete test suite to ensure all tests and linters pass.
• I have thoroughly tested my code changes to ensure they work as expected, including adding additional tests for new functionality.
• I have written clear and descriptive commit messages.
• I have provided a detailed description of the changes in the pull request, including the problem it addresses, how it fixes the problem, and any relevant details about the implementation.
• I have ensured that the code is well-documented and easy to understand.