feat: use blob_exists endpoint before uploading contract blob

[hal3e]

Summary

While working on the total_fee and total_gas PR I noticed that we rely on the error message to check if a blob is already included. Refactored the code to use the blob_exist endpoint.

Checklist

• All changes are covered by tests (or not applicable)
• All changes are documented (or not applicable)
• I reviewed the entire PR myself (preferably, on GH UI)
• I described all Breaking Changes (or there's none)