fix: validate non-empty flag key in Variable<T>

[jonathannorris]

Summary

Validates key in Local Variable<T> and VariableAsync<T>, throwing ArgumentException for null or empty keys.

Motivation

Invalid keys can reach the WASM bucketing engine and trigger AssemblyScript aborts. Repeated failures can corrupt WASM heap state and eventually cause Wasmtime runtime failures. Failing fast at the SDK boundary prevents this path and gives callers a clear exception.

Null defaultValue

Null is valid input for JSON variables, so this also removes the defaultValue null check from DevCycleCloudClient, where it already existed. Variable<T>.DetermineType called variableValue.GetType(), which threw NullReferenceException on a null default; it now falls back to typeof(T).

[Copilot]

Pull request overview

This PR adds fail-fast input validation to the Local client’s Variable<T>/VariableAsync<T> APIs to prevent null/empty flag keys (and null defaultValue) from reaching the WASM bucketing engine, aligning Local behavior with the existing Cloud client guards.

Changes:

• Added ArgumentException guard for null/empty key and ArgumentNullException guard for null defaultValue in DevCycleLocalClient.Variable<T> and VariableAsync<T>.
• Added MSTest coverage for null/empty key for both sync/async paths, plus a sync-path null defaultValue test.
• Normalized using System; line (removed prior BOM/formatting artifact) in the test file.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
DevCycle.SDK.Server.Local/Api/DevCycleLocalClient.cs	Adds key/defaultValue validation to Local Variable and VariableAsync to prevent invalid inputs reaching WASM.
DevCycle.SDK.Server.Local.MSTests/DevCycleTest.cs	Adds tests for null/empty key behavior and null defaultValue behavior (sync path).

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

Suppressed comments (2)

DevCycle.SDK.Server.Local/Api/DevCycleLocalClient.cs:382

• ArgumentException is thrown without specifying the parameter name. Providing the paramName improves diagnostics (e.g., "(Parameter 'key')") without changing the exception type or message.

            if (string.IsNullOrEmpty(key))
            {
                throw new ArgumentException("key cannot be null or empty");
            }

DevCycle.SDK.Server.Local/Api/DevCycleLocalClient.cs:313

• ArgumentException is thrown without specifying the parameter name. Providing the paramName improves diagnostics (e.g., "(Parameter 'key')") without changing the exception type or message.

This issue also appears on line 379 of the same file.

            if (string.IsNullOrEmpty(key))
            {
                throw new ArgumentException("key cannot be null or empty");
            }

[JamieSinn]

this is/was gated at the WASM layer before - so I'm 50/50 on this - but I do agree it's a good safety net.