status-cache: add shuttle tests

[alessandrod]

Next chunk split out from #3796

This only adds tests, doesn't change any logic.

[Copilot]

Pull Request Overview

This PR adds Shuttle tests to the status cache module for concurrent behavior testing. Shuttle is a deterministic testing framework for finding concurrency bugs by exploring different thread interleavings.

• Adds conditional compilation support for shuttle testing with feature gates
• Implements three shuttle test scenarios covering concurrent operations on the status cache
• Updates CI configuration to run the new shuttle tests

Reviewed Changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated no comments.

File	Description
runtime/src/status_cache.rs	Adds shuttle-specific imports, deterministic key indexing, and comprehensive shuttle test module
runtime/src/serde_snapshot/status_cache.rs	Conditionally imports Mutex from shuttle vs std based on feature flag
runtime/Cargo.toml	Adds shuttle-test feature and optional shuttle dependency
ci/test-shuttle.sh	Extends CI script to run shuttle tests for runtime module

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.2%. Comparing base (617d477) to head (b91e8c9).
⚠️ Report is 2061 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8275   +/-   ##
=======================================
  Coverage    83.2%    83.2%           
=======================================
  Files         836      836           
  Lines      366600   366604    +4     
=======================================
+ Hits       305201   305233   +32     
+ Misses      61399    61371   -28     

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[apfitzge]

minor comment about whether we should be using shuttle's Arc in a spot we're not currently (but using the other shuttle sync types)