FileSystem/Temp persistence: GetAllKeysAsync returns wrong keys for double/DateTime keys under non-invariant cultures (key 1.5 comes back as 15 in de-DE) #46

What is wrong

The code that turns a key into a file name and the code that turns a file name back into a key use different cultures.

• Writing: FileSystemPersistenceProvider.GetFilePath (Essentials.PersistenceProviders.FileSystem/FileSystemPersistenceProvider.cs:233) and the same method in Essentials.PersistenceProviders.Temp/TempPersistenceProvider.cs:257 both call key.ToString(), which formats with the current culture:

  string fileName = PersistenceProviderUtilities.GetSafeFileName(key.ToString()!) + _serializationProvider.FileExtension;

• Reading: GetAllKeysAsync (FileSystem line 191, Temp line 188) calls PersistenceProviderUtilities.TryConvertToKey. For any type other than string, Guid and int, that uses Convert.ChangeType(value, typeof(TKey), CultureInfo.InvariantCulture) (Essentials/PersistenceProviderUtilities.cs:180), which parses with the invariant culture.

In a culture whose decimal separator or date order differs from invariant, a stored key is listed back as a different key. That key does not exist, so ExistsAsync and RetrieveAsync on it miss.

Second consequence: because the file name depends on the culture, data stored under one culture cannot be found when the process runs under another (1.5 is stored as 1,5.json under de-DE and looked up as 1.5.json under en-US).

Failure scenario

CultureInfo.CurrentCulture = new CultureInfo("de-DE");
var p = new FileSystemPersistenceProvider<double>(new NativeFileSystemProvider(), new JsonSerializationProvider(), dir);
await p.StoreAsync(1.5, "hello");
var keys = await p.GetAllKeysAsync();

Output:

files: 1,5.json
keys: 15
Exists(15)=False Retrieve=<null>

Expected: keys: 1.5, and retrieving that key returns "hello".

With DateTime keys, new DateTime(2026,3,4,5,6,7) is listed as 2026-04-03T05:06:07 (day and month swapped).

How it was verified

A scratch console app referencing the FileSystem persistence, Native filesystem and Json serialization projects, run against HEAD, produced the output above.

Suggested fix / acceptance criteria

• Format keys with the invariant culture when building file names (IFormattable.ToString(null, CultureInfo.InvariantCulture) or Convert.ToString(key, CultureInfo.InvariantCulture)), using a round-trippable format ("R"/"O") where it matters. Share one helper between the FileSystem and Temp providers so they cannot drift.
• Add a test that sets CurrentCulture to de-DE, stores double and DateTime keys, and asserts that GetAllKeysAsync returns exactly the stored keys and that RetrieveAsync finds them after CurrentCulture is switched to en-US.