central_systest_blobs: generalize the GCS helpers to fetch and upload arbitrary objects

[itamar-starkware]

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[cursor]

PR Summary

Low Risk
Test-only refactor in cende_blob_regression_test.rs with no production or runtime path changes.

Overview
Refactors the Cende blob regression test’s GCS utilities so downloads and uploads are keyed by an explicit object path instead of a blobs-only generation helper.

fetch_raw_blobs_at_generation is replaced by fetch_raw_object, which takes any GCS object name. upload_new_object centralizes create-only uploads (if_generation_match: 0) with caller-supplied object path and string body. bump_generation_and_store_blob_file and fetch_blob_file now compose those helpers with blobs_object_path and normalized JSON; generation discovery and local generation file updates are unchanged.

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[itamar-starkware]

• apollo_integration_tests: verify Patricia storage proofs of persisted witnesses in flow tests #14905
• apollo_integration_tests: assert witness-carrying cende blobs reach the recorder in flow tests #14918
• apollo_integration_tests: validate cende blob witness fields in the dummy recorder #14904
• apollo_integration_tests: assert witnesses persisted for committed heights in flow tests #14903
• central_systest_blobs: publish a decoded commitment-infos companion beside the blobs #14917
• central_systest_blobs: carry real witness fields in the regression blobs #14910
• central_systest_blobs: generalize the GCS helpers to fetch and upload arbitrary objects #14920 👈 (View in Graphite)
• workspace,ci: remove the os_input feature (make it unconditional) #14902 : 2 other dependent PRs (#14907 , #14909 )
• main

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[yoavGrs]

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