[codeandsolder]

Fixes #2818.

Summary

• cache each multi-level backend's effective CacheMode in an async OnceCell
• reuse the cached mode on writes instead of calling Storage::check() per write
• skip levels known to be read-only while preserving the configured write-error policy
• preserve underlying write-error sources when adding level context

A normal server startup still performs the existing storage capability check before serving. Required probe failures keep their existing startup-failure behavior; tolerated failures remain best-effort.

Validation

• full Rust 1.99 library suite: 577 passed, 0 failed
• focused Rust 1.99 regressions verify a backend mode is checked once across multiple writes and tolerated check failures are not re-probed per write
• cargo fmt and git diff --check
• Mozilla Rust 1.91 exact CI Clippy gate passes

[codecov-commenter]

Codecov Report

❌ Patch coverage is 99.23077% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 78.87%. Comparing base (edeffeb) to head (3c1c8a4).

Files with missing lines	Patch %	Lines
src/cache/multilevel.rs	98.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2910      +/-   ##
==========================================
+ Coverage   78.82%   78.87%   +0.05%     
==========================================
  Files          75       75              
  Lines       45394    45512     +118     
==========================================
+ Hits        35781    35899     +118     
  Misses       9613     9613              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.