Add new feature flag constants for collection bulk decryption

[JaredScar]

🎟️ Tracking

https://bitwarden.atlassian.net/browse/PM-38906

📔 Objective

Utilize SDK for decryption and utilize decryptMany methods so only 1 SDK call is used

[github-actions]

🤖 Bitwarden Claude Code Review

Overall Assessment: APPROVE

This PR adds two feature flag string constants (CollectionBulkDecryptWithFailures and CollectionAdminBulkDecrypt) to FeatureFlagKeys in src/Core/Constants.cs in support of moving collection bulk decryption to the SDK. The change is minimal and self-contained.

Code Review Details

No findings. The new constants follow the existing conventions in FeatureFlagKeys, and declaring feature flag constants ahead of client-side consumption matches the established pattern in this file (e.g., the sibling PM35153CollectionSdkDecryption flag is likewise declared without server-side references). Constant declarations do not require unit tests.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.49%. Comparing base (8a720b6) to head (4fcf3a7).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7963      +/-   ##
==========================================
+ Coverage   61.45%   61.49%   +0.03%     
==========================================
  Files        2245     2253       +8     
  Lines       98855    99088     +233     
  Branches     8946     8959      +13     
==========================================
+ Hits        60754    60935     +181     
- Misses      35952    36002      +50     
- Partials     2149     2151       +2     

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