[PM-40047] Detect blob ciphers via format_version probe

[shane-melton]

🎟️ Tracking

https://bitwarden.atlassian.net/browse/PM-40047

📔 Objective

The SDK now stores blob-encrypted cipher payloads as JSON, which starts with { and collides with the leading-byte heuristic previously used to tell blob from legacy field-level CipherData. This replaces that heuristic in Cipher.IsDataBlobEncrypted() with a top-level format_version property probe (via Utf8JsonReader, so large payloads aren't fully materialized), matching the SDK. Tests updated. Paired with the sdk-internal change for PM-40047.

[github-actions]

🤖 Bitwarden Claude Code Review

Overall Assessment: APPROVE

Reviewed the replacement of the leading-byte heuristic in Cipher.IsDataBlobEncrypted() with a format_version top-level property probe via Utf8JsonReader, plus the aligned test updates. The new implementation correctly parses only top-level properties, handles malformed JSON via catch (JsonException), and the extracted HasTopLevelProperty helper cleanly separates the probe mechanism from the semantic check. Tests cover legacy, empty, invalid, and blob shapes.

Code Review Details

• ❓ : Semantics inversion for non-JSON Data — confirm no cipher was persisted in the earlier opaque (non-JSON) blob format, otherwise those rows would now hit an uncaught JsonException during deserialization.
  • src/Core/Vault/Entities/Cipher.cs:40

[codecov]

Codecov Report

❌ Patch coverage is 89.47368% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.99%. Comparing base (f0f04eb) to head (ef56ef9).
⚠️ Report is 34 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Core/Vault/Entities/Cipher.cs	89.47%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7930      +/-   ##
==========================================
+ Coverage   61.34%   65.99%   +4.64%     
==========================================
  Files        2244     2249       +5     
  Lines       98673    99074     +401     
  Branches     8912     8961      +49     
==========================================
+ Hits        60531    65382    +4851     
+ Misses      36006    31443    -4563     
- Partials     2136     2249     +113     

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