Fix Frends.JSON.QuerySingle compliance findings: net8, static class, CancellationToken, error handling

[Copilot]

Resolves 5 task analyzer findings (FT0004, FT0007, FT0011, FT0015×2) plus framework and project metadata requirements for Frends.JSON.QuerySingle.

Changes

Frends.JSON.QuerySingle.csproj

• Target framework: net6.0 → net8.0
• Version bump: 1.3.0 → 1.4.0

QuerySingle.cs

• JSON class made static (FT0004)
• CancellationToken added as required parameter (FT0007)
• Logic wrapped in try/catch delegating to ErrorHandler.Handle(options)

Definitions/Options.cs (FT0015)

• Added ThrowErrorOnFailure (bool, default true)
• Added ErrorMessageOnFailure (string, default "")

Definitions/Result.cs (FT0011)

• Added Error property of new Error class (Message, AdditionalInfo)

Helpers/ErrorHandler.cs (new)

• Standard Frends error handler: rethrows on cancellation, throws wrapped exception or returns failed Result based on ThrowErrorOnFailure

Frends.JSON.QuerySingle.UnitTests.csproj

• Target framework: net6.0 → net8.0

Tests

• Existing tests updated for new CancellationToken parameter and Exception (vs JsonException) throw type
• New ErrorHandlerTests.cs covering throw-on-failure, return-failed-result, and custom error message paths

// New signature
var result = JSON.QuerySingle(input, new Options { ThrowErrorOnFailure = false }, CancellationToken.None);
// On error: result.Success == false, result.Error.Message contains details

// Or throw on failure (default)
var result = JSON.QuerySingle(input, new Options { ErrorMessageOnFailure = "My error" }, CancellationToken.None);
// Throws Exception("My error", innerEx) on failure