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"

[thiyaguk09]

I’ve shifted to state-based testing as suggested. I also took the opportunity to clean up the sample script—adding proper await handling and ensuring it doesn't immediately delete the contexts it just created. This allows us to assert against the 'truth' in GCS metadata. Ready for another look!