Fix nullable struct deserialization across buffer boundaries

[eiriktsarpalis]

Streaming deserialization can throw JsonException for valid JSON when a buffer boundary follows a null member inside a nullable struct. ReadStack.Push() clears IsContinuation when restoring the deepest suspended frame, causing NullableConverter<T> to treat the member token as a null nullable value. Require ObjectState.None before taking that fast path.

Adds boundary sweeps, short-read and buffer-growth checks, and direct PipeReader tests for segmented buffers, one-byte flushes, and truncated input, with reflection and source-generated metadata.

On Windows, all 96 targeted cases per framework passed on net11.0 and net481; reverting the fix produces 48 failures per framework. All six System.Text.Json test projects passed with zero failures and 17 existing skips.

Resolves #110450

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