Storage → Test connection verifies conditional writes - #144

Resolves #141

Problem

• testConnection reported success the moment a provider accepted the credentials, never checking the conditional writes sync's compare-and-swap depends on
• Providers that reject conditional writes, or accept If-None-Match: * and silently ignore it, passed the test then either refused to sync or let concurrent edits overwrite each other with no error

Changes

• Added a conditional write probe that runs after the credential check, writing and rereading a throwaway object under the reserved prefix to confirm the provider enforces If-None-Match: * and returns an ETag
• Failed the connection test with a clear message when a provider ignores, rejects, or cannot support the writes sync needs
• Covered each failure mode with unit tests, with the happy path exercised against real storage

Why

• A connection test that passes for providers that lose edits is worse than none; this makes it answer the question it claims to, before a vault ever trusts the bucket

Greptile Summary

Adds a storage compatibility probe that:

• Verifies providers enforce conditional object creation.
• Confirms successful reads return an ETag.
• Deletes the temporary probe object without allowing cleanup rejection to replace the probe verdict.
• Adds unit coverage for supported providers and each compatibility failure mode.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

_{Reviews (2): Last reviewed commit: "Address comment around error handling" | Re-trigger Greptile}