Fix incorrect field offsets when parsing ETW events with fixed-count array fields

[Copilot]

ETW events using TraceLoggingUInt32FixedArray, TraceLoggingFloat32FixedArray, or any other fixed-count TraceLogging array type produced corrupted values for all fields following the first fixed array field. WPA parsed these correctly; TraceEvent did not.

Root cause

In RegisteredTraceEventParser.TdhEventParser.ParseFields, the fixed-count array path set arraySize = fixedCount (element count, e.g. 3) and passed that as the size argument to ArrayPayloadFetch. This stored the element count as the byte size on PayloadFetch.Size. Since OffsetOfNextField uses PayloadFetch.Size directly to advance past the field, every subsequent field was off by fixedCount * elementSize - fixedCount bytes (e.g., off by 9 bytes for a 3-element float32 array).

Fix

Replace the ArrayPayloadFetch call with FixedCountArrayPayloadFetch when fixedCount != 0. This existing helper correctly computes size = elementCount * element.Size before forwarding to ArrayPayloadFetch — the same pattern already used in EventPipeMetadata.cs.

// Before
propertyFetch = DynamicTraceEventData.PayloadFetch.ArrayPayloadFetch(
    arrayFieldOffset, propertyFetch, arraySize, fixedCount, projectCharArrayAsString: false);

// After
if (fixedCount != 0)
    propertyFetch = DynamicTraceEventData.PayloadFetch.FixedCountArrayPayloadFetch(
        arrayFieldOffset, propertyFetch, fixedCount, projectCharArrayAsString: false);
else
    propertyFetch = DynamicTraceEventData.PayloadFetch.ArrayPayloadFetch(
        arrayFieldOffset, propertyFetch, arraySize, 0, projectCharArrayAsString: false);