feat(storage): add Object Contexts samples and system tests

[thiyaguk09]

Description

• Add setObjectContexts.js to demonstrate CRUD and deletion of contexts.
• Add getObjectContexts.js to show retrieval of structured metadata.
• Add listObjectsWithContextFilter.js to demonstrate server-side filtering.
• Implement comprehensive system tests in files.test.js covering presence, absence (-), and existence (:) filter operators.
• Ensure samples use correct 'contexts' field with 'custom' map structure.

Checklist

• I have followed guidelines from CONTRIBUTING.MD and Samples Style Guide
• Tests pass: npm test (see Testing)
• Lint pass: npm run lint (see Style)
• Required CI tests pass (see CI testing)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This pull request is from a branch created directly off of GoogleCloudPlatform/nodejs-docs-samples. Not a fork.
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new sample directory, and I created GitHub Actions workflow for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

Note: Any check with (dev), (experimental), or (legacy) can be ignored and should not block your PR from merging (see CI testing).

[gemini-code-assist]

Code Review

This pull request adds new Node.js code samples for Google Cloud Storage to demonstrate retrieving, setting, and listing object contexts, along with a corresponding system test. The review feedback suggests correcting a comment that inaccurately describes a filtering operation and fixing a typographical error.

[snippet-bot]

Here is the summary of changes.

You are about to add 3 region tags.

• storage/getObjectContexts.js:28, tag storage_get_object_contexts
• storage/listObjectContexts.js:28, tag storage_list_object_contexts
• storage/setObjectContexts.js:28, tag storage_set_object_contexts

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[chandra-siri]

similar comment on this test file as #4272 (comment)

"Test the State, not the String"