State persistence architecture documentation

[brooke-hamilton]

Description

Adds a new architecture document describing the state persistence subsystems in Radius (database, secret store, message queue).

Changes

• docs/architecture/state-persistence.md — New document covering the three pluggable state persistence subsystems: database.Client, secret.Client, and queue.Client. Includes Mermaid diagrams, interface descriptions, implementation details, and data flow explanations.

Type of change

• This pull request is a minor refactor, code cleanup, test improvement, or other maintenance task and does not change the functionality of Radius (issue link optional).

Contributor checklist

Please verify that the PR meets the following requirements, where applicable:

• An overview of proposed schema changes is included in a linked GitHub issue.
  • Yes
  • Not applicable
• A design document PR is created in the design-notes repository, if new APIs are being introduced.
  • Yes
  • Not applicable
• The design document has been reviewed and approved by Radius maintainers/approvers.
  • Yes
  • Not applicable
• A PR for the samples repository is created, if existing samples are affected by the changes in this PR.
  • Yes
  • Not applicable
• A PR for the documentation repository is created, if the changes in this PR affect the documentation or any user facing updates are made.
  • Yes
  • Not applicable
• A PR for the recipes repository is created, if existing recipes are affected by the changes in this PR.
  • Yes
  • Not applicable

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 50.99%. Comparing base (209adb7) to head (7aa3d97).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11284      +/-   ##
==========================================
+ Coverage   50.98%   50.99%   +0.01%     
==========================================
  Files         679      679              
  Lines       43174    43174              
==========================================
+ Hits        22012    22017       +5     
+ Misses      19040    19036       -4     
+ Partials     2122     2121       -1     

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