Add IScopedAlternateLookup for scoped caches

[Copilot]

Summary

• add a new net9-only IScopedAlternateLookup API and expose it from IScopedCache
• implement scoped alternate lookup wrappers for ScopedCache and AtomicFactoryScopedCache
• add scoped alternate lookup unit coverage and default interface method tests

Validation

• dotnet format BitFaster.Caching/BitFaster.Caching.csproj
• dotnet build BitFaster.Caching/BitFaster.Caching.csproj -f net9.0
• dotnet test BitFaster.Caching.UnitTests/BitFaster.Caching.UnitTests.csproj -f net9.0
• parallel_validation

[coveralls]

coverage: 98.969% (-0.1%) from 99.11% — copilot/add-iscopedalternatelookup into main

[bitfaster]

@copilot add a soak test to verify this works as expected with concurrent callers. Soak test will cover all branches in the private ScopedGetOrAdd method.

[Copilot]

Added a concurrent soak test in ScopedCacheSoakTests that races alternate lookup ScopedGetOrAdd against TryRemove and asserts the returned lifetime stays alive under contention, covering the retry path in the private helper. Addressed in a3558be.

[bitfaster]

@copilot initialize the cache with the same size parameter as the other tests, 1 is an invalid size. Do not remove values as part of the test. Instead, make the test match WhenSoakScopedGetOrAddValueIsAlwaysAlive but use the alternate lookup.

[Copilot]

Updated the soak test to use the standard cache capacity and to mirror the existing scoped get-or-add soak pattern via alternate lookup, without removing values. Addressed in 99d95c4.

[bitfaster]

@copilot declare a char[] buffer variable and format j into the buffer to avoid allocating a string in the loop.

[Copilot]

Updated the soak test to reuse one char[] buffer per worker thread via Threaded.Run(threadCount, r => ...), so the key formatting no longer allocates in the hot loop. Addressed in 32705c1.

[bitfaster]

@copilot declare the key on this line and delete the array of keys. This will avoid capturing the keys array in the lambda.

[Copilot]

Updated it to declare the per-thread char[] buffer inside the worker lambda and removed the outer keys array, so the lambda no longer captures that array. Addressed in 330d9bf.