Limit eager ExtensionObject decode recursion

[kevinherron]

Motivation

• Prevent a denial-of-service vector where dynamic eager decoding recursively decodes nested ExtensionObject bodies without honoring the configured recursion budget, causing stack or heap exhaustion.
• Keep existing behavior of eagerly decoding known/expected dynamic types where safe, but ensure a shared recursion budget prevents unbounded nested decodes.

Description

• Add a thread-local recursion guard EAGER_DECODE_DEPTH and helper eagerlyDecodeExtensionObject(...) to FieldUtil so eager decoding of scalar, array, matrix, and Variant-contained ExtensionObjects checks EncodingLimits.getMaxRecursionDepth() and increments/decrements a local depth counter.
• Replace direct calls to xo.decode(...) in maybeEagerlyDecodeValue(...) with calls to eagerlyDecodeExtensionObject(...) so nested eager decodes share the same per-thread depth budget.
• Add a regression test eagerExtensionObjectDecodeHonorsMaxRecursionDepth and helper nestedStructWithStructureScalarFields(...) in DynamicTypeSerializationTest that constructs nested ExtensionObjects and verifies eager decoding halts at the configured depth and leaves deeper values opaque.

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Codex Task