chore: Add machine account token prefix feature flag

[maxkpower]

🎟️ Tracking

SM-2093, SM-2094

📔 Objective

Adds the Sm2093MachineAccountTokenPrefix feature flag (sm-2093-machine-account-token-prefix) to FeatureFlagKeys. This is infrastructure only — no behavior changes yet. It exists so the token-generation change in the stacked PR (#8369) can ship dark and be enabled independently.

[github-actions]

🤖 Bitwarden Claude Code Review

Overall Assessment: APPROVE

This PR adds a single feature flag constant, Sm2093MachineAccountTokenPrefix (sm-2093-machine-account-token-prefix), to the Secrets Manager section of FeatureFlagKeys in src/Core/Constants.cs. The key string follows the established <ticket>-<description> kebab-case convention, and neither the constant name nor its value collides with any of the 163 existing entries. No behavior is gated by the flag in this change, which matches the stated infrastructure-only intent; consumption lands in the follow-up PR. No security, correctness, or breaking-change concerns identified.

Code Review Details

No findings.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.49%. Comparing base (7673949) to head (0c9f051).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8368   +/-   ##
=======================================
  Coverage   64.48%   64.49%           
=======================================
  Files        2480     2480           
  Lines      106110   106110           
  Branches     9627     9627           
=======================================
+ Hits        68429    68435    +6     
+ Misses      35341    35337    -4     
+ Partials     2340     2338    -2     

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