fix: getProviders recognizes application/json accept header

[Nkovaturient]

PR #155

fix: content routing provider data handling and test cases

Description

Fixed the content routing implementation to ensure proper handling of provider data and JSON content types. All test cases are now passing across different environments (Node.js, Browser, WebWorker).

Key improvements:

• Added proper Content-Type: application/json headers
• Fixed provider data JSON formatting
• Added routing initialization in tests
• Fixed multiaddrs format handling in assertions

Test Results:

• Node.js: 23 passing, 2 pending
• Browser: 25 passing
• WebWorker: 23 passing, 2 pending

Note: The 2 pending tests are related to cache TTL functionality, which is expected behavior.

Notes & open questions

None - all test cases are passing as expected across environments.

Change checklist

• [✅] I have performed a self-review of my own code
• [✅] I have made corresponding changes to the documentation (updated test cases with proper JSON formatting)
• [✅] I have added tests that prove my fix is effective
• • [✅] Followed IPFS/Helia eslint practices