Validate JSON matrix dimensions

[kevinherron]

Motivation

• The JSON decoder accepted attacker-controlled Dimensions arrays and constructed Matrix objects without validating dimensions, which could lead to uncontrolled allocations and an OutOfMemoryError (DoS).
• The binary decoder performed per-dimension and total-length checks but the JSON decoder lacked equivalent validation for the flat Array + Dimensions representation introduced for compact JSON matrices.

Description

• Add a new validateMatrixDimensions(Object flatArray, int[] dimensions) helper in OpcUaJsonDecoder that rejects missing Array/Dimensions, negative dimensions, per-dimension sizes exceeding EncodingLimits.getMaxMessageSize(), overflow/over-limit products, and mismatched product vs flat array length.
• Call validateMatrixDimensions(...) in all JSON matrix construction paths: the Variant multi-dimensional array path, decodeMatrix(...), and decodeStructMatrix(...).
• Add unit tests to OpcUaJsonDecoderTest that assert oversized, negative, and mismatched Dimensions are rejected for Variant, built-in Matrix, and structured Matrix decoding.

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Codex Task