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

[jonathannorris]

Summary

• Add input validation for key and defaultValue at the top of both Variable<T> and VariableAsync<T> in the local client
• Throws ArgumentException for null/empty key, ArgumentNullException for null defaultValue
• Brings the Local client into alignment with the existing Cloud client guards (DevCycleCloudClient.cs:101-109) and the Java/Python SDKs

Motivation

When a null/empty flag key reaches the WASM bucketing engine, the empty target falls through to eventQueue.ts:122 and triggers throw new Error('Event missing target to save aggregate event'). AssemblyScript compiles that throw to abort() + unreachable, which wasmtime catches and rethrows as LocalBucketingException.

The trap unwinds without running AS's normal cleanup, leaving objects allocated in the trapped frame stranded in WASM linear memory. After enough traps the AS heap state corrupts, and on the .NET host the wasmtime Rust runtime eventually panics internally with a stack-pointer assertion in traphandlers/backtrace.rs:250.

Cross-SDK comparison shows .NET is the only host without this guard:

SDK	Empty-key guard?	Behaviour after many traps
.NET (Local)	No	Wasmtime Rust panic at ~100K iters
.NET (Cloud)	Yes (ArgumentException)	n/a
Java	Yes (IllegalArgumentException)	JVM survives, heap silently corrupted
Python	Yes (ValueError)	CPython survives, all calls silently default after ~178 iters

Behaviour change

Previously Variable(user, "", defaultValue) returned a defaulted Variable<T> after logging a WASM error. Now it throws ArgumentException immediately. Same for null key, and ArgumentNullException for null defaultValue. This matches the behaviour of every other entry point.

Tests

5 new tests in DevCycleTest.cs:

• Variable_NullKey_ThrowsArgumentException
• Variable_EmptyKey_ThrowsArgumentException
• VariableAsync_NullKey_ThrowsArgumentException
• VariableAsync_EmptyKey_ThrowsArgumentException
• Variable_NullDefaultValue_ThrowsArgumentNullException

All 57 existing tests still pass.

[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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