Remove in-memory state/runtime implementations; add pass-through API contract tests

[Copilot]

Previous changes incorrectly added in-memory .NET implementations for state interfaces and runtime contexts. FlinkDotNet is a pass-through translator to Flink Java — state management and runtime contexts live in the Java runtime, not in .NET.

Removed (12 files, ~1800 lines)

• State implementations: InMemoryValueState, InMemoryListState, InMemoryMapState, InMemoryReducingState, InMemoryAggregatingState
• Runtime implementations: ProcessContext, KeyedProcessContext, OnTimerContext, WindowContext, ListCollector, ResultFuture, SourceOutput, JsonSerializationSchema, JsonDeserializationSchema, JsonSimpleVersionedSerializer
• Associated tests: InMemoryStateImplementationTests.cs, RuntimeImplementationTests.cs

Added: FlinkApiContractTests.cs (76 tests)

• API surface validation for all state interfaces (IValueState, IListState, IMapState, IReducingState, IAggregatingState)
• Context and function interface contract tests matching Java Flink API
• User-defined function integration tests using mocked runtime (the correct pattern)
• Guard tests that fail if anyone re-introduces in-memory implementations

The correct usage pattern — user code implements functions, Flink Java runtime provides context:

// User implements the function interface
public class MyProcessFunction : IProcessFunction<int, string>
{
    public Task ProcessElementAsync(int value, IProcessContext ctx, ICollector<string> @out)
    {
        // ctx and @out are provided by Flink Java runtime, not .NET
        @out.Collect($"Processed: {value} at {ctx.Timestamp}");
        return Task.CompletedTask;
    }
}

// In tests, mock the runtime-provided interfaces
var mockCtx = new Mock<IProcessContext>();
var mockCollector = new Mock<ICollector<string>>();
await function.ProcessElementAsync(42, mockCtx.Object, mockCollector.Object);

Original prompt

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This section details on the original issue you should resolve

<issue_title>Are there still some parts that have not been fully developed?</issue_title>
<issue_description>Many of the interfaces mentioned in the documentation have not been implemented in v1.0.1.For example, ValueState, etc.</issue_description>

<agent_instructions>As @jerrypotti pointed out, many of the interfaces mentioned in the documentation have not been implemented in v1.0.1.For example, ValueState, etc. Please rescan the entire repo and implement all interfaces mentioned which aren't implemented</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@devstress Hi @jerrypotti, thanks for testing this out. It is still an early date version, since I don't have any feedback, so it is just an experimental version. I will continue from your request.

• Fixes Are there still some parts that have not been fully developed? #456

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