Throw SerializationException for null values in dense collections

[sugmanue]

Both codegen and dynamic schema deserialization paths now reject null values in non-sparse lists and maps with a SerializationException instead of silently skipping or corrupting data.

What behavior changes?

• Deserialization (codegen path): Previously, null values in dense lists/maps were silently skipped. Now throws SerializationException.
• Deserialization (dynamic schema path): Previously, null values in dense lists were coerced to the string "null", and null in dense maps flowed through unchecked, causing NPE later during serialization. Now throws SerializationException.
• Sparse collections: Behavior unchanged, null values are preserved as expected.

Why is this change needed?

A customer reported an NPE in the MCP server path when a response contained null values in a dense map. The null was silently accepted during deserialization, then caused a NullPointerException inside ContentDocument.serializeContents when the result was later accessed via getMember. The fix fails fast at deserialization time with a clear error message.

How was this validated?

• Dynamic schema tests (SchemaGuidedDocumentBuilderTest): 7 new tests covering dense throw, sparse preserve, validation on setMemberValue, and the customer's nested map NPE scenario.
• Codegen golden file test (sparse-vs-dense-collections): Verifies generated SharedSerde emits the correct throw/preserve logic for dense vs sparse collections.
• Codegen integration test (DenseSparseNullTest): 4 tests exercising generated builders at runtime with null values in dense/sparse lists and maps.
• XML codec tests updated: StringList in the test model marked @sparse since those tests exercise null-in-list behavior.

What should reviewers focus on?

• SchemaGuidedDocumentBuilder.java; ListConsumer and MapConsumer null handling (the core fix for dynamic schemas).
• ListGenerator.java / MapGenerator.java; template changes for codegen (the ${^sparse}throw...${/sparse} pattern).
• PluginTestRunner.java; removed discoverModels() to make golden file tests deterministic.

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